Hyper-V Replica provides asynchronous replication of virtual machines (VMs) from a Hyper-V host or cluster to a remote Hyper-V host or cluster to provide business continuity and fail-over recovery. This new, built-in feature tracks write operations on the source Host or cluster and replicates them to the destination host or cluster so that both VMs are in constant lockstep. If the local VM fails (or if an administrator manually fails-over) the remote replica assumes its place. All of this happens without having any specialized storage or networking hardware requirements.
The VHDX disk format is new to Hyper-V 3 and adds new capabilities and enhancements over previous versions:
A VHDX virtual hard disk can be as large as 64 TB
Support for larger block sizes in VHDX virtual disks. This provides support for dynamic and differential disks, which in turn, allows for lets these disks attune to the needs of the workload
A 4-KB logical sector virtual disk for applications and workloads that support 4KB sectors
Conversion from a VHD to VHDX and back
Live Migration is now built into Windows Server 2012 (whereas before, you only had this capability when you used System Center Virtual Machine Manager with Quick Storage Migration). Live Migration in Windows Server 2012 enables an administrator to migrate live VMs between stand-alone Hyper-V 3 hosts without requiring the use of any shared storage. The benefit of this is that the VMs can be moved while keeping the VMs online and available to clients, thus ensuring no downtime. You can perform a live migration through a GUI interface or through a PowerShell cmdlet.
As part of the live migration process, you can move different VM components to new locations on the destination host. This gives you a great deal of flexibility when performing a live migration