With XenServer it is possible to create backups of VMs, even if they are running. The process is as follows:

Search for the uuid of the VMs to backup

First look for the uuid of the VMs to backup. We don’t want to backup the control domain itself, so we add is-control-domain=false to the vm-list command:
[codesyntax lang="bash"]

xe vm-list is-control-domain=false

[/codesyntax]

Create a snapshot of each (running)

Now we create a snapshot of the VMs we want to backup, replacing the uuid one by one with the ones we found with the previous command. Also replace the name of the snapshot if desired:
[codesyntax lang="bash"]

xe vm-snapshot uuid=8d6f9d81-95b5-2ffb-4ecc-b5e442cc5c22 new-name-label=gb-r7n2-snapshot

[/codesyntax]

This command has a return value: the uuid of the created snapshot. Then we transform the snapshot into a VM to be able to save it to a file, replacing uuid with the return value of the previous command:
[codesyntax lang="bash"]

xe template-param-set is-a-template=false ha-always-run=false uuid=4efd0392-8881-176c-012e-a56e9cb2beed

[/codesyntax]

Save the snapshot to file

In the next step we save the snapshot to a file, replacing uuid with the snapshot uuid and providing a meaningful filename:
[codesyntax lang="bash"]

xe vm-export vm=4efd0392-8881-176c-012e-a56e9cb2beed filename=gb-r7n2-snapshot.xva

[/codesyntax]

Remove the created snapshot

In the final step we delete the snapshot:
[codesyntax lang="bash"]

xe vm-uninstall uuid=4efd0392-8881-176c-012e-a56e9cb2beed force=true

[/codesyntax]

Source: http://www.jansipke.nl/creating-backups-of-running-vms-in-xenserver/