The leaked Windows 8 build 7989 also contains Hyper-V 3.0 and a new virtual hard drive format, VHDX.



From WinRumors:

Both features have been uncovered by enthusiasts digging around the leaked build. Robert McLaws has detailed the support in a blog post on Monday. Windows 8 build 7989 includes the following Hyper-V 3.0 features:

Storage
Virtual Fibre Channel Adapter
Storage Resource Pools
New .VHDX virtual hard drive format (Up to 16TB + power failure resiliency)
Memory/Processor Enhancements
Support for more than 4 cores! (My machine has 12 cores)
NUMA – Memory per Node, Cores per Node, Nodes per Processor Socket
Networking Enhancements
Hardware Acceleration (Virtual Machine Queue & IPsec Offload)
Bandwidth Management
DHCP Guard
Router Guard
Monitor Port
Virtual Switch Extensions
Network Resource Pools

  Windows 8: Hyper-V 3.0 and new VHDX virtual hard drive format discovered