XenServer is an enterprise-class, cloud-proven, virtualization platform that delivers all of the critical features needed for any server and datacenter virtualization implementation.
XenServer is a Linux distribution that is based on the Xen hypervisor, the Linux kernel, and the CentOS Linux distribution and user tools.
free version is limited
windows management client
XCP-ng
(+) Nice Windows GUI Tool XCP-ng Center
(-) No WEB UI
(+) Xen Orchestra independent Web GUI: centralised management tool. Capable to manage multiple locations, moving VMs between locations.
delta backups, continuous replication
(-) Thin provisioning only on local EXT3/4 fs, NFS, File, XOSAN (VDI files)
Block based devices only with THICK provisioning (local LVM, iSCSI, HBA)
(-) experimental ZFS
Proxmox
(+) simple
virtualization
(+) KVM virtualization
(+) native support for LXC
(-) no docker support
(+) Debian based - possible to do lots of unsupported things from OS
(+) run on any HW
(+) GPU/PCI passthrough
(+) Clustering - one web interface to manage multiple nodes (but not separate locations)