Q: Are Windows NT 4 and Windows 2000 guest OSs supported on Windows Server 2012 Hyper-V?

A: Windows NT 4 and Windows 2000 are no longer supported by Microsoft, and so can't be supported on Hyper-V. Windows Server 2012 Hyper-V actually dropped integration service support for Windows 2000, which means it's no longer possible to use synthetic devices (such as network and storage) and there are no services to integrate with Hyper-V.

The net effect is the performance would be poor compared to an OS using synthetic devices, because emulated devices would have to be used. The recommendation would be to run Windows 2000 virtual machines (VMs) on a Windows Server 2008 R2 Hyper-V server, which still has integration service support for Windows 2000.

Another (unsupported) option to try could be taking the integration services from Hyper-V 2008 R2 and installing on the Windows 2000 VM, then running on the Windows Server 2012 Hyper-V. This would give some synthetic device support and some integration with Hyper-V but would be completely unsupported by Microsoft. Longer term recommendation would be to migrate to a newer, supported OS as soon as possible.

For Windows NT 4, there really is no support. The legacy processor compatibility mode that helped NT 4 run on Hyper-V is gone in Windows Server 2012 Hyper-V, and emulated devices would definitely have to be used. However, the performance might meet what you need so this could be worth a try.