In this case I’m hosting the VM on a fast server and trying to access the display on another system (a laptop).
One way to do it is by simply SSHing with X forwarding and running KVM like so:
qemu-system-x86_64 -boot d -cdrom ubuntu-18.04.2-live-server-amd64.iso -m 8192 -enable-kvm
This by default uses a terminal window, but it’s quite slow.
Another option is to start the KVM machine in nographic mode and enable a VNC server:
qemu-system-x86_64 -nographic -vnc :5 -boot d -cdrom ubuntu-14.04.6-desktop-amd64.iso -m 8192 -enable-kvm
then on the desktop system use a vnc client to connect to the magic port: