The Xen and KVM feature updates aren't the most exciting, but there are some changes worth mentioning.
On the Xen front, it has switched over to the new CPU hotplug mechanism, supports driver_override in picback, and requires vector callbacks for HVM guests. The individual changes explained here.
With KVM the changes are more notable and includes a number of ARM fixes, MIPS prep work for supporting MIPS EVA (Enhanced Virtual Addressing) host kernels, MIPS SMP fixes, small fixes and optimizations for PowerPC, and various minor enhancements to the KVM x86 code.
Those wishing to learn about the KVM changes for Linux 4.9 can see this pull request.