A couple of weeks ago Xen.org released Xen maintenance release 4.0.1.
It’s primarily for bug fixing but it also introduces a few new capabilities.
First of all the default pvops kernel is now 2.6.32 (Xen 4.0 used kernel 2.6.31). Secondarily, they pygrub boot loader now support loading para-virtualized guests using GRUB2 configuration files. But more importantly, Xen 4.0.1 introduces Remus support, the Xen fault-tolerance package, for pvops dom0 kernels.
Xen 4.0.1 is expected to be included in the Xen Cloud Platform (XCP) as the project further matures. It still uses Xen 3.4.2