I assume, you use LVM volumes for your XEN guests. I’m not going to use “xm migrate” here, the method used works by dd’ing the LVM volume over to the new Dom0, so make sure, you have a fitting LVM volume in place on your destination system.
I recommend you to stop the machine you’re going to move (or you could consider to create a LVM snapshot). Anyways, if you know nothing will change you can try it with the running machine (I did this once, and it resulted in a fsck upon boot but without any further problems).
With this one you can dd the LVM volume to the new host:
dd if=/dev/x bs=1M | ssh username@remote-server "dd of=/dev/y bs=1M"
To check the status of the copyjob, open a new console and issue (note: the USR1 signal lets dd print some infos):
watch -n 5 "killall -USR1 dd"
To finish the move, copy the XEN host config file to the new system:
scp /etc/xen/hostconfig username@remote-server:/etc/xen/hostconfig