The Linux 3.9 kernel already is set to feature a lot and when it comes to Xen virtualization we now have an idea of what's coming.
Konrad Rzeszutek Wilk of Oracle has posted about the Xen changes he's been queuing up for the Linux 3.9 kernel. Among the noteworthy work is a Xen ACPI memory hot-plugging driver to dynamically add/remove system memory to Xen guests, and a CPU hot-plugging driver for adding/removing CPUs/cores to guests using ACPI. Additionally, Xen PVH presents support for Para-Virtualized Hardware extensions in this next release.
More of the Xen patches being planned for the Linux 3.9 kernel can be found on the kernel mailing list.