In strict terms of its hypervisor functionality, Hyper-V Server uses very little memory. However, it is inextricably linked to the management operating system, which requires memory just like any other operating system.
Memory for the hypervisor and the management operating system is not directly assigned by an administrator (although a reserve can be set). Virtual machines will be allocated memory according to their demands and the hypervisor, and management operating system will have access to whatever is left. It is not possible to entirely squeeze either of these functions out through virtual machine overload; Hyper-V will refuse to start any virtual machine that would cause complete memory starvation.
However, it may allow you to drain memory from the management operating system to the point that its performance is impacted. Take care in the way that you design and allocate memory.
The management operating system, whether you are using Windows...