@faceplant It is xen-api vm paramter. i fount it is a problem of ubuntu 18.4 grub always set gfxpayload = keep, but VM gdm can't with it. and set gfxpayload = text or do not set it in /boot/grub/grub.conf. gdm works OK.

Turn default template platform:viridian parameter to true, and install again works fine. Or changing original Ubuntu 18.04 desktop VM platfrom:viridian to true is also effected.