When you create the KVM VM in Proxmox, you should be able to select SeaBIOS (aka legacy BIOS). With that selected it should "just work".
It doesn't matter if the Promxox host is UEFI (or not), but the guest VM needs to be setup to not use UEFI (I'm still on v6.x but it's default on that).
You should be able to select SeaBIOS (aka legacy BIOS)
When you create the KVM VM in Proxmox, you should be able to select SeaBIOS (aka legacy BIOS). With that selected it should "just work".
It doesn't matter if the Promxox host is UEFI (or not), but the guest VM needs to be setup to not use UEFI (I'm still on v6.x but it's default on that).