New to KVM virtualization on Linux with the 3.12 kernel is support for nested EPT and PV-TicketLocks. The first is support for nested Extended Page Tables while the PV-TicketLocks code is ported to KVM from the Xen code.
The new Paravirtualized Ticket Spinlocks replaces earlier ticket lock code for virtualization and is meant to better deal with vCPUs. The work on PV-TicketLocks has mostly come from IBM for both Xen and KVM. More information on PV-TicketLocks including benchmarks can be found from the Xen.org blog.
Aside from this modest feature work, there's also KVM bug-fixes for all major supported architectures. More details on the KVM virtualization code churn for the 3.12 merge window can be found via this Git commit.