Clustering, migration, high availability, backups, Ceph integration, virtual networks as of recent, can do containers as well as VMs to name a few off-hand. The web interface is optional, too. You should check out their webpage for more.
re: backups, their other product "proxmox backup server" integrates really well with the backup system they have in place. It can be run in a container on the proxmox host itself.
KVM is just kernel side of things, it's not full vmm by itself, you always need some userspace application too. Firecracker, Qemu, Cloud-Hypervisor are some vmms built on top of kvm.
While qemu is common way of using kvm, but running qemu directly is quite annoying. So you have stuff like libvirt and proxmox as wrappers around qemu.