I don't know what Proxmox is, I'll take a look.
ProxMox is a "package" of KVM. Unlike installing KVM or Xen on your own and needing to assemble all of the parts, it's a complete ecosystem package like vSphere is to ESXi. So you get the GUI, the web interface, the API, the file systems, the backup tools, and so forth. It's an "install and use" system, not a "pick each piece and put it together yourself" system. Nothing wrong with putting your own together, but unless you are getting some extreme advantage from carefully knowing and tuning every piece that you select, better to go with something already assembled, known, and supported rather than reinventing a really well built and free wheel.
ProxMox is basically the XCP-NG of the KVM world.