This tutorial for installing macOS Monterey Developer Beta has been adapted for Proxmox from Kholia’s OSX-KVM project and Leoyzen’s OpenCore configuration for KVM. You can get the full sourcecode of my OpenCore release on my GitHub here.
Since Monterey is still in closed Developer Beta, you need to be an Apple Developer and have access to a Mac (or Mac VM) to download it. If you don’t have access to that, maybe you’d like to try my Big Sur tutorial instead while you wait for the public release.
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 Illegal Instruction crashes when apps/extensions attempt to use these missing instructions.
Modern AMD CPUs also support SSE 4.2 and will work with this guide.