I investigated on many forums and tried everything
Let me save you a LOT of time. Windows 2008 TAKES all available RAM. It caches it, when it's not in use. This is normal. It's because Hyper-V doesn't do transparent sharing of memory, so when you assign memory to a VM on Hyper-V you can't over commit memory. Why is this relevant?
Because BOTH products are Microsoft, and there really isn't a problem, you just have to be aware of what's going on. There is no way to get Windows 2008 to relinquish the memory, that's how it's designed.
I also notice that you said the memory in the VM is low, that means its not being used, that means you are giving WAY too much memory in Windows 2008 VM's, even for developers. We have developers also, and one BIG mistake most NEW ESX admins (and ones that aren't paying attention) VM's don't need as much RAM as physical hosts (too many reasons to list). Suffice to say, reduce the memory footprint for your VM's to 50%. We give no MORE than 2GB for any Windows 2008 VM and we use Visual Studio, Customer Relational programs, and an assortment of debugging tools, and 2GB is fine, still high in many cases. We have Windows 2003 VM's with 1GB, that's PLENTY for what they are doing. Don't believe the hype. Admins and VM ware are smarter than users, remember that.
They don't know how VM works (the users) so don't let them dictate what they need. Give them 2GB, you will reduce your host memory and they will still get what they need.
