Do you mean Hyper-V VMs and the Hyper-V host?
As then no, at the moment that's not directly possible, a client for Windows is still under planning and a Hyper-V integration is also not yet available. The prime client candidates for the first Proxmox Backup Server final 1.0 release will be Proxmox VE integration and Linux hosts.
You could backup Linux VMs running on Hyper-V from the inside, though.
As then no, at the moment that's not directly possible, a client for Windows is still under planning and a Hyper-V integration is also not yet available.