Against some rumors it is NOT possible (by now, which means in Beta and RC releases of Hyper-V) to over-commit physical memory assigned to virtual machines.
There might have been plans during Hyper-V development to allow that the total sum of virtual memory assigned to concurrently running VM, may exceed the total amount of available physical memory, but if that has been planed it has been dropped in releases of Hyper-V so far.
So a configuration like this:
- physical Host has 18 GB of real world RAM
- first VM is set to 6 GB of virtual RAM
- second VM is set to use 4 GB virtual RAM
- third VM is set to use 9 GB virtual RAM
which makes a sum of 19 GB of virtually assigned memory if all virtual machines are running at the same time, will result in the following screened error when trying to start the third VM (it doesn't matter which VM is started last/first):