Markus Pfeiffer reports success using Xen HVM to run DragonFly, which may be useful for any of you Xen users. He reports not being able to use more than 2 virtual CPUs, though Scott Tincman reports successfully using 4 (with qemu), so your mileage may vary.
Updated: noting qemu usage as Markus pointed out in comments.
Just a quick note that Scott is running qemu, not xen.