Specifically, are you talking about using "xm" - or something else? If you're talking about "xm", do you see the downtime as acceptable? And do you see the network load due to effectively the file transfer as being acceptable, or do you see an "rsync" being performed across multiple "xm" images and if so do you see the load on the source (Xen host) server being acceptable?
Using "xm" is not acceptable because there is downtime, obviously that doesn't fly.
Network load is unavoidable. That applies to every technology out there. Rsync is probably the best process for transporting snapshots off of the machine in question.