This tutorial for installing macOS Big Sur Final using OpenCore has been adapted for Proxmox from Kholia’s OSX-KVM project and Leoyzen’s OpenCore configuration for KVM. You can get the full sourcecode on my GitHub here.
I’ll assume you already have Proxmox 6 installed. You also need a real Mac available in order to fetch the OSK key.
Your Proxmox host computer’s CPU must support SSE 4.2, so for Intel your CPU must be at least as new as Nehalem, which was the first CPU generation to bear the “Core” i5/i7 branding. Older CPUs will cause the finder to repeatedly crash after installation completes (with an Illegal Instruction exception in the graphics code).
Modern AMD CPUs also support SSE 4.2 and will work with this guide.