Xen 4.4 presents ARM64 guest support, guest UEFI booting support with TianoCore, kexec support, upgrades to SeaBIOS and QEMU, PHV domU experimental support, improved SPICE support in libxl, multi-vector PCI MSI support, event channel scalability work, and many other changes. PVH support is a big Xen feature that also made it into the Linux 3.14 on the kernel-side, plus Xen 4.4 has other exciting improvements now that it's become a project of the Linux Foundation.
Those wishing to checkout the 4.4 details can see Xen's roadmap and the official 4.4 feature list. There isn't a guaranteed release date right now for Xen 4.4 final besides "when it's ready" this year. It does look like the final release may now be on approach though with the release candidates doing well and updates today for Xen 4.4-RC4.
Those wishing to see Xen 4.4 be a great success, being organized tomorrow by the Xen community is the Xen 4.4-RC4 test day. All the details for the 4.4-RC4 test day on Tuesday is documented by that Wiki page. When Xen 4.4 is out you can expect to see some new Linux virtualization benchmarks on Phoronix.