Tier-1 means the HyperVisior gots direct access to the Hardware. Tier-2 has just access to the OS, and the OS to the Hardware.
There is a special Windows Server 2012 HyperV Version available, which comes without GUI, that needs less Resources by itself to run. BTW: It's free!
With Windows 2016 there will come "Windows Nano Servers", it's a minimal OS with just Infrastructur Features like Hyper-V and Dockerization Service. It just could be managed with Powershell.
Optional there is XenServer (6.5sp1) Tier1 Hypervisor which is OpenSource and free.