Proxmox VE is a complete virtualization management solution for servers. You can virtualize even the most demanding application workloads running on Linux and Windows Servers. Checkout their page: proxmox.com
Due to lack of better backup functionality I prepared patches to support differential backups in Proxmox VE. These patches are in use for over a year.
Currently supported versions are 3.4, 4.1, 4.2, 4.3, 4.4, 5.0.