If the goal is to backup the VMs rather than the Citrix XenServer Dom0, I find that exporting each VM to a shared disk (NFS, iSCSI, whatever) is the easiest solution. If you need zero-downtime, then make a snapshot of the VM and then export it.
I did this specifically for the purpose of disaster recovery. Recovering individual files or datasets from within a VM is also possible but is slightly more of a hassle.
I have some scripts that do this that I'm willing to share.
BTW... Centos and Citrix XenServer are different enough that these kinds of questions should probably be asked on the XenSource mailing list or the Citrix forums. You can get potentially misleading advice otherwise.
-geoff
--------------------------------- Geoff Galitz Blankenheim NRW, Germany http://www.galitz.org/ http://german-way.com/blog/