The kernel-xen package is compiled with PAE support. This means that machines such as my Pentium-M laptop can not run Xen.
There is a very tricky situation with Xen on i386 & PAE. You basically have to have a matched stack, either all PAE, or all non-PAE - you can't mix & match. Since PAE is needed for machines with > 4 GB ram, and all current CPUs do now have PAE (even current laptop targetted CPUs), in FC6 the decision was made to go for PAE kernels. Unfortunately this excludes some older laptops without PAE.
Does matched stack mean that you need to compile all xen-relevant packages compiled with or without PAE support? If that's the case, then I understand the decision, but if it's not, then it would be better to have kernel-xen and kernel-xenPAE instead of what we currently have.
No, it's only the kernel-xen package that matters (that package includes both the kernel and the hypervisor, which are the only two components that care about PAE status.) But the PAE choice in paravirtualised guests MUST match that in the dom0 host.