Serial console works, but is ouput only.
Looks like it's being caused or triggered by ACPI PNP. As a temporary
workaround, use pnpacpi=off on the kernel command line (unless you actually need
it, of course).
Nice catch --- with pnpacpi=off, I can run a xen0 kernel and still capture
serial console from another box.
I've reproduced this problem in xen-unstable, so it looks like an upstream bug.
This should have been resolved with the upstream fix.