Log in to the host server.
1. List all vm-s and copy the vm uuid you want to backup.
xe vm-list is-control-domain=false
2. Create a snapshot of the vm.
xe vm-snapshot uuid=[uuid of vm] new-name-label=snapshotname
3. Convert the snapshot from a tmplate to a vm.
xe template-param-set is-a-template=false ha-always-run=false uuid=[uuid of snapshot]
4. Save the snapshot to a file.
xe vm-export vm=[uuid of snapshot] filename=filename.xva
5. Delete the snapshot.
xe vm-uninstall uuid=[uuid of snapshot] force=true
6. Now just copy the saved file to a backup server by ssh, ftp, nfs or any other method you prefer.
You can also save the file to nfs share directly in step 4.
