I am more familiar with VMWare products. They use the actual RAM installed to provide RAM to the VM. This is different than using a thumbdrive to increase RAM.
If you have 6 GB of installed RAM on your host and allocate 5 GB to a single VM. The host will only have 1 GB left for it's use.
Hyper-V functions quite similar. It has a dynamic memory setting. Until you understand it's affects, I would turn that off. Dynamic memory allocation can help you to automatically balance memory usage between multiple VM and host but, if you are using a memory hog app, it may cause problems with that VM or another, when fired up.