One physical machine, many independent computers. The idea underneath cloud, labs and modern IT.
A virtual machine is a complete computer (CPU, memory, disk, network card) that exists as files on a real one.
The hypervisor is the software that creates those machines and hands them slices of the real hardware.
The guest OS does not know. It boots, sees hardware, and behaves exactly as it would on metal.
This chapter: what a hypervisor is, the two types, and the two Type 2 products you will actually install.
Why an A+ technician cares
Every cloud service you have ever used runs in a VM. Every safe place to test a suspicious file is a VM. Every "can you support Windows 10 and 11 and Linux on one laptop" is a VM.
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The problem virtualization solves
Before VMs, one physical server ran one workload, and mostly sat idle.
One box, one job
A server running a single application typically used 5–15% of its CPU. The other 85% was paid for, powered, cooled, and wasted.
Slow to provision
Needing a new server meant purchasing, racking, cabling and installing. Weeks.
Fragile to test
Trying a patch meant trying it on the real thing, or buying a second real thing.
Hard to recover
A broken OS meant a reinstall from media, then reconfiguration from memory.
The shift
Virtualization turns a computer into a file. Files can be copied, moved, paused, duplicated, rolled back and deleted. Every advantage below follows from that one sentence.
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What a hypervisor actually does
It is a traffic controller for real hardware, handing time-slices and address ranges to guests that think they own the machine.
Scheduling. Real CPU cores are shared between guests, the way an OS shares one core between programs.
Memory mapping. Each guest sees an address space starting at zero. The hypervisor translates to real addresses.
Device emulation. The guest sees an ordinary disk and NIC; the hypervisor turns those into a file and a virtual switch.
Isolation. A guest cannot read another guest's memory. This is the property everything else depends on.
The term to keep straight
Host = the physical machine and the software running the VMs. Guest = the operating system inside a VM. Exam questions are frequently just testing whether you read that the right way round.
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Type 1 vs Type 2: the distinction that matters
The only real question: is there a host operating system between the hypervisor and the hardware?
How to answer this in one sentence
Type 1 IS the operating system. Type 2 RUNS ON an operating system. If you can minimise it and check your email, it is Type 2.
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Type 1 in the wild
Bare-metal hypervisors. What data centres and cloud providers run.
VMware ESXi
The long-standing enterprise standard. Managed at scale by vCenter.
Microsoft Hyper-V
A Windows Server role. Also present on Windows Pro, where it still installs beneath the OS.
KVM
Built into the Linux kernel. What most public clouds and our own OpenStack sandbox run on.
Xen
The open-source hypervisor behind a large share of early cloud infrastructure.
Faster, because there is no host OS to traverse for every instruction.
More stable, because there is no host OS to crash underneath the guests.
Not for a laptop. Installing one wipes the machine. It becomes the operating system.
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Type 2 in the wild
Hosted hypervisors. What you install on a computer you are already using.
Oracle VirtualBox
Free, open source, runs on Windows, macOS and Linux. The usual teaching choice.
VMware Workstation Pro
Now free for personal use. Stronger performance and snapshot tooling.
VMware Fusion
The macOS member of the same family, including Apple Silicon.
Parallels Desktop
macOS only, paid, tuned for running Windows smoothly beside macOS.
The trade you are making
Slower than Type 1 and dependent on the host staying up, in exchange for installing in five minutes on a machine you still use for everything else. For a technician bench, that trade is correct.
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Oracle VirtualBox up close
The one most students will install first, and the one most likely to appear in a lab.
Cost. Free under the GPL. The Extension Pack (USB 2/3, RDP, disk encryption) has separate licensing: free for personal use, not for commercial.
Guest Additions. Drivers installed inside the guest. Without them: fixed resolution, no shared clipboard, no drag and drop. Installing them is the fix for "the screen will not resize".
Disk format. VDI by default; also reads VMDK, VHD and HDD.
Snapshots. Full tree, unlimited branches.
The one students trip on
A brand-new VM boots to "FATAL: No bootable medium found". That is correct behaviour. It is an empty computer. You still have to attach an ISO to the virtual optical drive.
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VMware up close
Workstation Pro on Windows and Linux, Fusion on macOS. Same engine, same file formats.
Cost. Workstation Pro and Fusion Pro became free for personal use in 2024. Commercial use still requires a licence.
VMware Tools. The equivalent of Guest Additions, and the same first fix for resolution, clipboard and time-sync complaints.
Disk format. VMDK. Splittable into 2 GB chunks for transport across filesystems that dislike very large files.
Why people pay attention to it. A VM built here moves to ESXi and vSphere in production without conversion.
Choosing between them, honestly
For learning, VirtualBox: free everywhere, runs on everything, enormous documentation. For work that ends up on VMware infrastructure, Workstation: the file formats and the habits transfer.
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VirtualBox vs VMware side by side
They do the same job. The differences are licensing, polish and where the VM goes next.
Price
VirtualBox: free (Extension Pack restricted commercially). VMware Workstation/Fusion: free for personal use, licensed for commercial.
Platforms
VirtualBox: Windows, macOS (Intel), Linux, Solaris. VMware: Workstation on Windows/Linux, Fusion on macOS including Apple Silicon.
Performance
VMware is generally faster on 3D and heavy I/O. For a text-mode Linux guest, you will struggle to tell them apart.
Guest tooling
Guest Additions vs VMware Tools. Same purpose, not interchangeable. Install the one that matches the hypervisor.
Disk format
VDI vs VMDK. Both read the other; VMDK travels better into enterprise VMware.
Snapshots
VirtualBox: unlimited tree, free. VMware: unlimited tree, with a clearer manager UI.
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What the hardware has to provide
Virtualization is a CPU feature. Without it you get either a refusal or unusable speed.
CPU extensions. Intel VT-x or AMD AMD-V. Almost always present since roughly 2010, and frequently disabled in firmware.
64-bit CPU, and enough cores that host and guests are not fighting over one.
RAM is the usual ceiling. Guest RAM is taken from the host while the VM runs. 16 GB is a comfortable two-VM bench; 8 GB is one small guest.
Disk. Budget the full installed size of every guest, plus snapshot growth. SSD strongly preferred: VMs are I/O-bound.
Nested virtualization is a separate CPU feature, needed only when a VM must itself run VMs.
The error you will actually see
"VT-x is not available (VERR_VMX_NO_VMX)" or "This host supports Intel VT-x, but it is disabled." Neither is a broken install. Both mean: reboot into UEFI/BIOS and turn it on.
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Turning it on in UEFI/BIOS
The single most common blocker between a student and their first VM.
Reboot and enter firmware setup, usually Del, F2, F10 or F12 during POST.
Find it under Advanced, CPU Configuration, or Security. Named Intel Virtualization Technology, VT-x, SVM Mode or AMD-V.
Enable, save, exit. A full power cycle is sometimes required. A warm reboot may not apply it.
On Windows, Task Manager → Performance → CPU reports "Virtualization: Enabled" once it takes.
The conflict worth knowing
Hyper-V, WSL2, Windows Sandbox, Memory Integrity and Credential Guard all claim the virtualization extensions. When they hold them, VirtualBox and VMware run degraded or refuse. Modern versions cooperate, older ones do not. If a Type 2 hypervisor misbehaves on Windows, check what else is using the CPU extensions first.
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Anatomy of a virtual machine
Everything the guest believes is hardware, and what it really is on the host.
vCPU
Scheduled time on real cores. Assigning more vCPUs than you have cores makes guests slower, not faster.
vRAM
Reserved from host RAM while running. Over-allocate and the host swaps, and everything crawls.
Virtual disk
One file: .vdi, .vmdk, .vhdx. Dynamically allocated grows as used; fixed size is faster and pre-claims the space.
Virtual NIC
Plugged into a virtual switch. Its behaviour is entirely decided by the network mode.
Virtual optical drive
Where you attach the installer ISO. The reason a new VM will not boot.
Config file
The .vbox or .vmx describing the machine. The VM is this file plus its disk, nothing else.
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Virtual networking: four modes
The setting that decides whether the guest can reach the internet, the host, or nothing at all.
Pick by intent
Just needs updates: NAT. Must be reachable as a server: Bridged. Talking only to the host: Host-only. Detonating something dangerous: Internal, and confirm it, because "I thought it was isolated" is how a lab machine infects a network.
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Snapshots and why they are not backups
Freeze a machine's exact state, break it deliberately, and return.
What it stores. Disk state, plus memory and CPU state if taken while running, so a resume can land mid-application.
What it is for. The moment before a risky change: a patch, a driver, a registry edit, opening a suspicious file.
Why not a backup. A snapshot lives on the same disk, inside the same VM, and depends on the original. Lose the disk and both are gone together.
The cost. Every snapshot grows a delta file. A long chain silently consumes disk and slows the VM.
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What it is actually used for
The A+ objectives phrase these as use cases. They are also the reasons you will personally run one.
Sandboxing
Open the suspicious attachment somewhere that can be deleted. The single most valuable habit in this chapter.
Testing and development
Try the patch, the upgrade, the unfamiliar distribution, on a copy.
Legacy application support
The application that only runs on Windows 7 keeps running, inside a VM, on a supported host.
Cross-platform work
Run Linux on a Windows laptop, or Windows on a Mac, without dual-booting.
Training and labs
A whole network of machines on one computer. Break them; rebuild in minutes.
Server consolidation
Ten lightly-used physical servers become ten VMs on one well-used host.
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Security in both directions
A VM is a strong boundary and a real attack surface. Both are true.
Isolation is the feature. Malware in a guest cannot ordinarily touch the host or other guests.
VM escape is the exception: a hypervisor vulnerability that crosses the boundary. Rare, severe, and the reason hypervisors get patched.
Shared folders, shared clipboard and drag-and-drop are deliberate holes in that boundary. Turn them off before analysing anything hostile.
Network mode is a security control. Bridged puts a possibly-infected guest directly on the LAN.
VM sprawl. Forgotten VMs stay unpatched and keep their credentials. An unpatched machine is not safer for being virtual.