Linux Virtualization

The Linux virtualization stack has distinct layers, and most tools only talk to the one below them.

flowchart TB
  subgraph clients [Management tools]
    virsh
    vm[virt-manager]
    cockpit[Cockpit]
    vagrant[Vagrant · libvirt provider]
  end
  clients --> libvirt[libvirtd]
  libvirt -->|"starts one process per VM"| qemu[QEMU]
  qemu -->|"/dev/kvm"| kvm[KVM kernel module]
  kvm --> cpu[CPU with VT-x / AMD-V]

Layer	Role
KVM	Kernel module that runs guest code directly on the CPU using its virtualization extensions.
QEMU	Userspace process per VM that emulates the hardware (disks, network cards, display). Without KVM it can still emulate a whole CPU, also of other architectures, but slowly.
libvirt	Daemon with one API for VMs, networks and storage pools. Stores VM definitions as XML.
Clients	`virsh` (CLI), virt-manager (GUI), Cockpit (web) and others all go through libvirt, so they see the same VMs.

Checking support

Command	Expected result
`lscpu \| grep Virtualization`	`VT-x` or `AMD-V`. Otherwise enable it in the firmware.
`ls -l /dev/kvm`	The device exists and your user can access it.
`virt-host-validate`	Checks the host for KVM and libvirt requirements.

virsh

Command	Effect
`virsh list --all`	All VMs, including stopped ones.
`virsh start <vm>`	Starts a VM.
`virsh shutdown <vm>`	Asks the guest to shut down (ACPI).
`virsh destroy <vm>`	Pulls the plug. The VM is stopped, not deleted.
`virsh undefine <vm> --remove-all-storage`	Deletes the VM and its disks.
`virsh autostart <vm>`	Starts the VM when the host boots.
`virsh console <vm>`	Serial console; leave with `Ctrl`+`]`.
`virsh edit <vm>`	Edits the XML definition.
`virsh snapshot-create-as <vm> <name>`	Creates a snapshot.
`virsh domifaddr <vm>`	IP addresses of the VM.
`virsh net-list --all`	Virtual networks.