Proxmox VE is a packaging of Linux as an operating system. It is a distribution. Straight from the wikipedia page:
It is a Debian-based Linux distribution with a modified Ubuntu LTS kernel[7] and allows deployment and management of virtual machines and containers.[8][9]
Cool way to respond to a comment btw:
Am I taking crazy pills?
The VMs I’m running in Proxmox are also Linux, but that’s less interesting to me.