What you must know

This section contains basic information about concepts and objects used when managing Virtual Machines.

QEMU modes: KVM vs emulation

When using REVEN with QEMU VMs, you will have to switch between two virtual machine mode, because recording is restricted to full emulation mode:

ModeTechnologySpeedWhat we use it for
KVMVirtualization with hardware supportFastInstallation, heavy configuration
Emulation (TCG)Full software: code for each instructionSlowerRecording, simple configuration

From the point of view of the guest OS running, these two modes are completely different hardware. Going from one to the other is akin to moving a physical hard disk between two machines, and as a consequence requires properly shutting down the VM before making the switch.

Note: the same is true when changing most other hardware options, you must do it while the VM is shut down.


REVEN uses the native disk / live snapshot mechanism from QEMU which might differ from what users expect. Two different objects are called "snapshots", so to avoid any confusion we need to differentiate:

NameWhat it isSupport switching VM modes
Disk snapshotThe living VM's hard diskYes, by shutting the VM down
Live snapshotA frozen state of a running VM,
to be restored anytime
No: must be restored using the same
mode it has been created with

Live snapshots are very handy, because they provide a very quick way to restore a VM from a known good VM state, instead of having to wait for the VM to boot up. This is even more true in Emulation mode, where booting a VM takes a few minutes.

Finally, note that the above table is a simplification. Notably, live snapshots are stored within the qcow disk snapshot file itself, and this is reflected in the Project Manager's organisation.

You can now head over to the Adding VMs section.