One of the big features of Xen 4.3 includes initial ARM support via the ARMv7-A virtualization extensions as well as the ARMv8 architecture. The ARMv7 support has been tested on the Arndale Board, Samsung Chromebook, ARM Fast Models, and ARM Versatile Express. The ARMv8 testing for now was just done on ARM Fast Models until there's actual 64-bit ARM hardware available.
Xen 4.3 performance enhancements come in the form of NUMA scheduler affinity to benefit NUMA hardware, scalability improvements so physical RAM can now be up to 16TB in size rather than 5TB, and a limitation of 300 virtual CPUs was removed so Xen 4.3 can now accomodate 750 virtual CPUs.
Xen 4.3 also provides better security via vTPM enhancements and the XSM/Flask security subsystem.
Other features include tooling improvements, usability improvements, better power efficiency, Intel vAPIC support, support for newer VIA CPUs, ACPI v5 support, and many other features. This release does away with 32-bit and IA64 support.
More details on Xen 4.3 virtualization can be found via its Wiki feature list and release notes.