When a virtual machine running on Windows Server 2012 Hyper-V is booted over the network using PXE, only the legacy network adapter can be used. This legacy adapter is limited to 100 Mbps. The much faster synthetic NIC supporting 10 Gbps does not support PXE. This is not ideal when for example Citrix PVS is used to provision new virtual machines. This means two network adapters need to be configured for the virtual machine. See my post on this restriction here.
This restriction is solved in Windows Server 2012 R2. Server 2012 R2 Hyper-v allows virtual machines to be created using the current Generation 1 type, or using the new Generation 2.
Generation 2 virtual machines only support synthetic network interface cards. New is that the synthetic NIC does support PXE network boot.
However, Generation 2 virtual machines only support 64-bit editions of Windows 8, Windows 8.1, Windows Server 2012 and Windows Server 2012 R2 as guest operating systems.