You can do better than this: the CLI command (from outside your VM)
xe vm-export vm=(UUID-of-VM) full-path-to-output-file will export the whole image of your VM in XVA format, and this is perfectly suitable for backup. Also, it can be exported to different machines, to different hypervisors, and so on. It carries not just all of your disk contents, but the whole settings of your VM, network cards, multiple disks (if any) and so on.
Just remember that saving by means of CLI can only save to XVA format, which will need to be converted to OVF format via Xenconverter (read here). You can save directly to OVA from the Xencenter but, for some reasons I do not know, that can only be done via GUI, not CLI.