The Plan 9 kernel for Xen 3.0 in /n/sources/contrib/xen/xen3 now supports PAE mode, which is the default for distributions like Debian and virtual hosting providers like Amazon EC2.
PAE only really makes sense (if at all) on 32-bit machines with more than 4GB of RAM, but lacking such a machine myself I haven't been able to test for the case of physical page addresses exceeding 32 bits. If any volunteer with access to "big iron" running Xen 3 would like to try it out, it would be much appreciated.
