Since Proxmox is based on the Debian, you can try to protect it using our Veeam Agent for Linux.
Please review the System Requirements for the Agent in the User Guide here. Thanks!
I had a quick look on the Internet, and Proxmox does not appear to be a hypervisor, but rather a "vCenter" so to speak (management software). The only hypervisors that Proxmox can manage is KVM. So, are you basically asking for KVM hypervisor support? In that case, there's an existing topic about this already.