Operating Systems

Virtualization

Here’s the truth behind virtualization: an operating system is just a program that thinks it owns the hardware. Give it something that looks like hardware, and it will happily run – alongside a dozen others on the same machine. That simple trick underpins the cloud, CI pipelines and most server rooms on Earth.

What it is, in plain words

Virtualization means running a complete computer – CPU, memory, disks, network card – in software, on top of a real one. The software that does this is a hypervisor, and each pretend computer is a virtual machine (VM) with its own guest operating system.

There are two classic kinds of hypervisor:

Type 1 (bare metal) – runs directly on the hardware: KVM (inside Linux), Xen, VMware ESXi, Microsoft Hyper-V. Platforms like Proxmox VE package KVM with a web UI, storage and clustering.
Type 2 (hosted) – runs as an app on a normal OS: VirtualBox, VMware Workstation, Parallels, UTM.

Modern CPUs make this fast with hardware extensions – Intel VT-x / AMD-V for the CPU, EPT/NPT for memory, and VT-d / AMD-Vi (IOMMU) for handing real devices to guests. Guests use virtio paravirtual drivers to talk to the hypervisor efficiently instead of pretending to be old hardware.

A lighter cousin is containers (Docker, Podman, LXC): no separate kernel, just isolated processes sharing the host kernel. Somewhere in between sit microVMs like Firecracker, which boot a tiny VM in a fraction of a second.

Why it matters

Isolation – break a VM and the host is fine. Perfect for testing malware, new distros or risky upgrades.
Consolidation – one big server runs many small machines.
Snapshots – freeze a whole computer, experiment, roll back.
Portability – a VM image moves between machines and clouds.
The cloud itself – every AWS, Google Cloud or Hetzner server you rent is a VM (or a container in a VM).

An everyday analogy: a block of flats

A physical server is a plot of land. Without virtualization you build one house per plot. With it you build a block of flats: each tenant has their own front door, kitchen and locks (their own OS), but they share the foundations, the water main and the electricity (CPU, RAM, disks). The building manager – the hypervisor – makes sure nobody uses more than their share or walks into a neighbour’s flat. Containers are more like rooms in a shared house: lighter, cheaper, but you share the kitchen (the kernel).

flowchart TB
  subgraph VMs
    G1[Guest OS + apps] --- G2[Guest OS + apps]
  end
  subgraph Containers
    C1[app] --- C2[app]
  end
  VMs --> H[Hypervisor: KVM / Xen / Hyper-V]
  Containers --> K[Shared host kernel]
  H --> HW[Hardware with VT-x / AMD-V]
  K --> HW

A hands-on example: KVM on your own Linux box

Check your CPU supports it:

grep -cE 'vmx|svm' /proc/cpuinfo   # > 0 means yes
ls -l /dev/kvm                      # the KVM device should exist

Install QEMU and libvirt. On Void Linux:

sudo xbps-install -S qemu libvirt virt-manager
sudo ln -s /etc/sv/libvirtd /var/service/
sudo ln -s /etc/sv/virtlockd /var/service/
sudo ln -s /etc/sv/virtlogd /var/service/
sudo usermod -aG libvirt,kvm $USER

Quick and raw – boot an ISO straight from QEMU:

qemu-img create -f qcow2 disk.qcow2 20G
qemu-system-x86_64 -enable-kvm -m 4G -smp 2 -cpu host \
  -drive file=disk.qcow2,if=virtio \
  -cdrom void-live-x86_64.iso -boot d \
  -nic user,model=virtio-net-pci

Or the managed way, with virt-install (or the virt-manager GUI):

virt-install --name voidtest --memory 4096 --vcpus 2 \
  --disk size=20 --cdrom void-live-x86_64.iso --os-variant generic
virsh list --all
virsh snapshot-create-as voidtest clean "fresh install"

Snapshots are where virtualization pays for itself: try a risky change, and virsh snapshot-revert voidtest clean if it goes wrong.

Common misconceptions

“VMs are slow.” With hardware extensions and virtio, CPU-bound work runs at near-native speed. Disk and graphics are where overhead shows.
“Containers are lightweight VMs.” They share the host kernel. A Linux container can’t run a Windows kernel, and isolation is weaker.
“Installing two hypervisors is harmless.” On Linux, VirtualBox and KVM have long competed for VT-x. Recent kernels and VirtualBox 7.x relax this, but running both at once is still asking for trouble. Pick one.
“A VM is perfectly isolated.” Escapes are rare but real, and side channels like Spectre crossed VM boundaries. Patch the host.
“You need a server for Proxmox.” An old desktop with enough RAM makes a fine home lab.

Connections

VirtualBox – the friendliest desktop hypervisor.
Windows WSL – a VM disguised as a subsystem.
Operating Systems and Modern Operating Systems – what’s being virtualized.
Penetration Testing – labs are built from VMs.
Online VPS and Linode – renting someone else’s VMs.
Proxmox VE - Virtualization Management Platform