I installed Slackware 15 in Hyper-V.
After using: > mkinitrd -c -f ext4 -r /dev/sda1 -m hv_vmbus:hv_storvsc:mbcache:jbd2:ext4 -u -o /boot/initrd.gz -k 5.15.19 < I was able to successfully boot into Slackware.
But when using > startx <, I' getting errors:

xauth: file /root/.serverauth.1037 does not exist
Fatal server error: no screens found
xinit: giving up
xinit: unable to connect to X server: Connection refused
xinit: server error

My /var/log/Xorg.0.log has the following errors:

open /dev/dri/card0: No such file or directory
Screen 0 deleted because of no matching config section
no screens found

Anyone have any info on this - having the same issue.

Ok, found the solution: mount the slackware 15 CDROM on /mnt/cdrom, then as root run:

installpkg /mnt/cdrom/extra/xf86-video-fbdev/xf86-video-fbdev-0.5.0-x86_64-1.txz

Then startx and it should fire right up.

Basically the video driver needed for Hyper-V is not on the system by default.

You an do a similar thing using slackpkg to install from one of the online mirrors.