Virtualization: QEMU/KVM & libvirt

When you shut down a KVM host, what happens to the running VMs — and where does their state go?

What is the relationship between QEMU and KVM, and why do RHEL and SUSE lean on the pair for enterprise virtualization?

You want to run KVM guests on a cheap cloud VM you rented from a hosting provider. Why does that usually not work?

Beyond ordinary x86 emulation, what makes QEMU able to run software built for a completely different CPU architecture?

KVM is often called a hypervisor, but what does the KVM kernel module actually do — and not do — on its own?

How do you check from the shell whether a CPU supports the hardware virtualization KVM requires, and what does an empty result mean?

On Ubuntu, what does the kvm-ok command tell you that a raw /proc/cpuinfo grep does not?

What happens when you start a VM on a host whose CPU lacks (or has disabled) hardware virtualization?

KVM ships as three kernel modules — which one do you load, and what happens to the others?

Why can't you run VirtualBox and KVM at the same time on one host?

What command launches modern KVM VMs directly, why is RHEL the exception, and why is direct use discouraged?

Which libvirt tools cover the day-to-day KVM admin tasks, and how do they divide up?

What single process must be running before virsh or virt-manager can do anything, and how do remote connections reach it?

libvirt can run VMs at the session level or the system level — which do you pick for a server, and what's the security trade-off?

You run virsh as root, but your KVM guest can't read its own disk image or ISO. What subtlety about libvirt privileges explains this?

On Ubuntu, how does a non-root user gain the right to manage system-level VMs, and what really controls that access?

Where does a VM's XML definition live, and why should you never open it in a text editor directly?

Where does libvirt store disk images by default, and what must you do first to keep them somewhere else?

KVM does full virtualization needing no special guest drivers, so what do the optional virtio drivers buy you?

Why can a Linux guest use virtio immediately while a Windows guest needs a two-step install?

QEMU/KVM offers RAW and QCOW2 disk images. RAW is simpler and faster — so when is QCOW2 worth the trade-off?

Which disk image format does Virtual Machine Manager default to, and what happened to the third format QEMU once had?

With KVM's default NAT networking, what happens when you try to SSH from the host into the guest — and how does that differ from VirtualBox?

A KVM guest's screen is shown over VNC or Spice — what does Spice give you that VNC doesn't?

What firmware and hardware does a QEMU/KVM guest need before Windows 11 will install, and what packages provide them?

Which virsh commands cover a VM's lifecycle, and how do you identify the machine you're acting on?

Inside virsh, how do you switch between user-level and system-level connections, and what's the syntax trap for a remote host?

In virsh, what's the practical difference between suspending a VM and saving it?

What's the difference between virsh destroy and virsh undefine, and which one endangers your data?

How do you make a KVM VM start automatically at host boot, and which VMs does this actually work for?

You run virsh vncdisplay on a VM and get nothing back. What are the two possible reasons, and how do you connect instead?

What three things does virt-clone do to copy a VM, and what state must the source be in first?

After virt-clone, the new VM boots but collides on the network or (on RHEL) won't run at all. What cleanup did you miss?

Instead of hand-editing a cloned VM, what does virt-sysprep automate for you?

You run virt-viewer as a normal user and it can't find your system-level VM. What flag fixes it?

Why does bridge setup instructions for KVM differ depending on your distribution?

After building a bridge for KVM, guests still can't route traffic properly. What kernel setting is easy to forget?

Why can a KVM VM only use a network bridge when it runs at the system level, never at the session level?

On a rented root server, a bridged KVM setup can get your whole server locked by the provider. What's the cause and the fixes?

You want to edit config files inside a VM's disk image without booting it. When is that safe, and when will it corrupt the filesystem?

How do you mount an individual partition from inside a RAW disk image, and how do you clean up afterward?

What does libguestfs give you that manually kpartx-mounting an image does not?

Which libguestfs tools would you reach for to inspect an image's disk usage, filesystems, installed OS, and file contents?

How do the virt-tar-out, virt-tar-in, and virt-make-fs tools move data in and out of disk images?

You have a VirtualBox VDI disk and want a compressed QCOW2 for KVM. Which command converts it, and what flag controls compression?

You need a VM's disk bigger. What does virt-resize require you to set up first, and what does --expand do?

A VM's image file on the host is far bigger than the data df reports inside the guest. Why, and how do you reclaim the space?

When you want a top-style live view of all your VMs at once, which tool gives it?

With libvirt's default user-mode networking, what IP range do guests get and why can't LAN machines reach them?

To make a KVM guest visible on the LAN you set up a network bridge — what does the bridge actually connect, and why is it more work than in VirtualBox?