Comment # 8 on bug 1196289 from Charles Arnold

(In reply to Ulrich Windl from comment #0)
> When updating the kernel, older version(s) are kept.
> However when updating Xen, only the most recent version is kept.
> So if the current Xen has a problem, there is no easy fix.
> (Furthermore it seems booting older Snapshots is broken for the Xen case; I
> also think there was a bug report/enhancement request for that recently)

This last point at least is addressed in a feature request.

PED-74 [XEN] Make working currently broken Xen snapshot boot