There are exceptions to this such as Proxmox which can actually be added to an existing Debian install. I must admit that when I first encountered it I didn't expect much more than a glorified toy. However it is so much more than that and they do a really good job with the software and the features. If anybody is on the fence about it I recommend giving it a go. If you do, I recommend using the ISO to install, pick ZFS as the filesystem (much much more flexible), and run pbs (proxmox backup server) somewhere (even on the same box as an lxc host with zfs backed dir).