Creating Nested VMs with KVM

Objective

Install the KVM hypervisor stack on Linux, create and manage virtual machines using virsh and virt-install, and understand the libvirt architecture.

Tools & Technologies

KVM
qemu
libvirt
virsh
virt-manager
virt-install

Key Commands

egrep -c '(vmx|svm)' /proc/cpuinfo

apt install qemu-kvm libvirt-daemon-system

virsh list --all

virt-install --name vm1 --memory 1024 --disk size=10

Architecture Overview

graph TD HOST[Host Machine] --> KVM[KVM Kernel Module] KVM --> QEMU[QEMU Emulator] QEMU --> LV[libvirt daemon] LV --> VM1[VM 1] LV --> VM2[VM 2] LV --> VM3[VM 3] LV --> API[API / virsh / virt-manager] style HOST fill:#1a1a2e,stroke:#00d4ff,color:#e0e0e0 style LV fill:#1a1a2e,stroke:#00ff88,color:#e0e0e0 style API fill:#1a1a2e,stroke:#ffd700,color:#ffd700

Step-by-Step Process

Verify CPU Support

KVM requires hardware virtualization extensions. Check before installing.

# Count virtualization-capable CPUs
egrep -c '(vmx|svm)' /proc/cpuinfo  # >0 means supported

# More detail
lscpu | grep Virtualization
kvm-ok  # ubuntu: apt install cpu-checker

Install KVM Stack

Install QEMU, KVM, libvirt, and management tools.

sudo apt install -y \
  qemu-kvm libvirt-daemon-system \
  libvirt-clients bridge-utils \
  virt-manager virtinst

# Add user to libvirt group
sudo usermod -aG libvirt $USER
newgrp libvirt

# Start and enable libvirt
sudo systemctl enable --now libvirtd

Create a VM with virt-install

Use virt-install to create a VM from an ISO.

sudo virt-install \
  --name testvm \
  --ram 1024 \
  --vcpus 1 \
  --disk size=10 \
  --location /path/to/ubuntu.iso \
  --os-variant ubuntu22.04 \
  --network default \
  --graphics none \
  --extra-args 'console=ttyS0'

Manage VMs with virsh

virsh is the command-line interface to libvirt.

virsh list --all           # list all VMs
virsh start testvm         # start VM
virsh shutdown testvm      # graceful stop
virsh destroy testvm       # force stop
virsh console testvm       # serial console
virsh snapshot-create-as testvm snap1
virsh snapshot-revert testvm snap1
virsh undefine testvm --remove-all-storage

Challenges & Solutions

Nested virtualization requires enabling in host BIOS and VMware/VBox settings
Permission denied to virsh — user not in libvirt group, relogin required

Key Takeaways

Use snapshots before major changes — virsh snapshot-create-as is instant
virt-manager provides a GUI when a desktop is available