Dunno. Haven't had the time to look at the docs and run Xen yet. On top of that my Linux dev box broke down this week which means I haven't got access to Qemu right now. If it supports USB it will be in the docs. One thing I've been using a lot is the Qemu suspend. Halting the guest O.S. and then saving memory contents (savevm command) and running the image later with the vm flag gives you almost instant access to the guest O.S.. Works great in most situations.