Whenever I log into those servers, I find that times are faster than host's and these time gaps are getting bigger.
Usually, commercial Hypervisors such as XenServer, VMware and Hyper-V keep time synchronization between the host and the guest through installing a program called "guest tool".
However, In the case, they not provided any tool in Xen (not XenServer)
When I was stuck in the same situation on Linux, I resolved time drift in the case of Linux Operating system by setting NTP.
So, It would recommend that NTP is best solutions for Windows HVM, too.
This is a hint from the online document.
To enable NTP on Windows, it is able to change it by configuring the Windows Time Service. Time service is registered as Windows Service automatically and it is also provide the way of changing in command prompt.
By running command prompt, At first, Check current time source of the computer.
Then, It adds a ntp server to the list.
Windows Time Service" in services panel.