Updated November 16, 2016
Hyper-V virtual machines show no data about Memory Utilization.
Hyper-V only provides correct memory metrics for virtual machines which use Dynamic Memory. Virtual machines with Static Memory will show no data about Memory Utilization, only the amount of assigned memory. This also applies to all virtual machines running on Windows Server 2008 without SP, which does not have the capability of providing Dynamic Memory.
If possible, turn on Dynamic Memory.
Windows Server 2008 without SP does not provide Dynamic Memory. For virtual machines running on this hypervisor, the memory utilization graph does not contain any data points. For these virtual machines only information about the amount of assigned memory is available on the Summary view of the virtual machine.
On other versions, enable Dynamic Memory to make sure that the memory utilization graph contains correct values.