Key changes to Linux 3.15 with regard to KVM are:
- The PowerPC KVM code now supports transactional memory within guests.
- MIPS KVM improvements, but that is expected to be more exciting for the Linux 3.16 kernel as QEMU is soon gaining MIPS KVM support.
- KVM on x86 has optimizations for debug registers since they are triggered when running some Windows games. There's also other Windows guests enhancements.
- KVM x86 also has a fix/workaround for Apple Mac OS X guests.
- Intel Broadwell CPU instruction set extensions are now exposed as well as Intel MPX.
- Asynchronous page faults for KVM s390 along with IRQ improvements that speed-up s390 VirtIO devices.
More details can be found via the Linux 3.15 pull request by Paolo Bonzini.