Is It Really Useful?
As well as providing virtualization on a single machine, Xen allows VMs to be migrated. In the simplest case, you could save your OS state to a flash drive and carry it around with you.
A more complicated arrangement involves a guest operating system that uses network-attached storage in a cluster. Xen allows these to be migrated between cluster nodes on-the-fly.
If you have 100 servers with load spikes at different times, then you could run them as 100 Xen VMs on (for example) 30 cluster nodes, with the ones under heavy load getting an entire machine to themselves and the remainder sharing a small number of nodes. The Xen team demonstrated this migration using a VM running a first-person shooter game server, and none of the players noticed when the server moved nodes.