Beginning with kvm-76, you can now assign a unique machine identifier to your kvm guest machine during startup using the –uuid option. This will probably be used by management tools based on libvirt to manage a database of already existing kvm guest machines since libvirt already uses the uuid format to identify virtual machines. The uuid format is in hexadecimal and of the format 8x-4x-4x-4x-12x where x is a digit so 8x is 8 hex digits and 4x is 4 hex digits etc. The new option allows you to do two things: 

1. specify a uuid during startup and
2. a monitor command for displaying the uuid of your kvm guest machine.

The syntax for the uuid startup option is as follows:

qemu-system-x86_64 –hda windows.img –m 400 –uuid 12345678-1234-1234-1234-123456789abc

From within the monitor, you can now issue the info uuid to display the unique identifier for your kvm guest machine. For example, to display the uuid of the guest started above:

QEMU 0.9.1 monitor - type 'help' for more information

(qemu) info uuid
12345678-1234-1234-1234-123456789abc