I've used this method successfully to move virtual hosts about:
Create the proper volumes using LVM on the new dom0, and mount them on /mnt.
Then login to the virtual host while it's running.  Update the kernel,
and make any necessary changes.  Login to the old dom0 and copy across
the /etc/xen/client.conf file, making any necessary changes.  Make sure
rsync is installed on the client and on both dom0's.  Then on the client,

     cd /
     rsync -az -e ssh  --sparse  --exclude /afs --exclude /proc --exclude /sys ./ root@newdom0:/mnt/

Then shutdown the client.  As soon as it is stopped, on olddom0, mount the
client's partitions on /mnt, then

     cd /mnt
     rsync -az -e ssh --sparse --delete-excluded ./ root@newdom0:/mnt/

Then on the new dom0, unmount the partitions mounted on /mnt, and
'xm create client.conf'.  The client should be down less than a
minute if you do this properly.

Useful commands on xen servers:
   ll /etc/pas* /etc/grou*
   (cd /etc;cp -p group.real group;cp -p passwd.real passwd)