With the release of macOS Catalina 10.15, this tutorial is now obsolete! Please check out the new Catalina tutorial here.
This tutorial for installing macOS Mojave has been adapted for Proxmox 5.4 from Kholia’s GitHub project for installing into vanilla KVM. There is more documentation there which will help out with enabling extra features and diagnosing problems!
I’ll assume you already have Proxmox 5.4 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).
Apparently modern AMD CPUs also support SSE 4.2 and can be used with this guide without any modification (maybe Bulldozer and certainly Ryzen), but I haven’t tested this myself.