On Ubuntu ish systems (not sure about base debians), you can get full KVM support in qemu by installing the "qemu-kvm" package right out the repos and then star

On Ubuntu ish systems (not sure about base debians), you can get full KVM support in qemu by installing the "qemu-kvm" package right out the repos and then starting your VMs with the "-enable-kvm" switch. You may also need to get EFI working in qemu to support OSX EFI bootloader, which you can get by installing the "ovmf" package right out the repos, and then adding the "-bios OVMF.fd" switch to your qemu command (OVMF.fd being the firmware file used by qemu to do EFI, which the "ovmf" package seems to install near /usr/share/qemu/OVMF.fd). I'm not sure about GPU passthrough, but with full KVM support enabled, you will immediately notice a pretty huge difference in the performance of the VM.