Among the work in QEMU 0.15 is many ARM enhancements, the x86 target has Xen support merged, KVM x86 now supports VIA CPU features, improved qemu-img performance, qcow2 has improved snapshot performance, the IDE emulation support now supports the TRIM command, and much more.
The Xen support in QEMU, of course, is prominent after previously the Xen-QEMU support was carried via a fork of QEMU. Now there's upstream QEMU support for the Xen virtualization target. It was just with the Linux 3.0 kernel that it offered full support for Xen virtualization in the mainline tree.
More of the changes, download links, and other information about the just-released QEMU 0.15 can be found on the project Wiki.