I'm on Z17 pro + Nvidia. Hope this saves someone else all the frustration I went through to solve this. 
No KVM shows up on Flatpac and the 3d acceleration is missing on both snap and flatpac. I didn't try the apt version because the repro hasn't been updated in 2 yrs.
Note: virt-manager & virtualbox work without issue on my system.
First thing I did was uninstall gnome boxes and all associated files, then applied the fixes below.
1. NO KVM ERROR
First, if this is your first time using virtualization on your system, you need qemu and all that jazz... (Heck, everyone with issues should run this first line to ensure you have all required dependencies.) Open a terminal and enter this....
sudo apt -y install qemu-kvm libvirt-daemon-system libvirt-clients bridge-utils virtinst libosinfo-bin
Next, run each of these one line at a time
sudo systemctl start libvirtd
sudo systemctl enable libvirtd
sudo setfacl -m user:$USER:rw /var/run/libvirt/libvirt-sock
Now install gnome boxes via terminal
flatpak install flathub org.gnome.Boxes
Then run this command...
flatpak override --user --env=LIBVIRT_DEFAULT_URI=qemu:///session org.gnome.Boxes
You should now be able to install an iso in gnome boxes error free
This fix came from here: https://github.com/flathub/org.gnome.Boxes/issues/99
2. MISSING 3D ACCELERATION
This one was painful to fix. I patched this bit by bit until it worked. Fingers crossed it works for you too.
In your terminal run this...
sudo usermod -aG video,render $USER
Important: Ensure all VM's (boxes) are shut down in gnome boxes. Right click on the VM and select "Preferences". Scroll down to "Edit Configuration" and click the pencil. Select an editor.
FIRST MAKE A COPY OF THIS XML and save it. (If this fix doesn't work you will be able to restore the XML with your copy.)
Scroll down and find the "video" section. Replace ALL the text inside those tags with this...
<video>
<model type='virtio' heads='1' primary='yes'>
<acceleration accel3d='yes'/>
</model>
</video>
Find the "graphics" section and change it to this...
<graphics>
<graphics type='spice' autoport='no'>
<listen type='none'/>
<gl enable='yes' rendernode='/dev/dri/renderD128'/>
</graphics>
Hit "Save" in your editor, exit preferences and run your VM. It should now run with acceleration in the background and your VM should be fast and smooth.
Note: If this works for you, you will need to change the XML of each new iso you install.
If this does not work for you, open the error log in preference and scoll to the bottom. The last few lines should say what this issue is.
Good luck and happy VM'ing!! Cheers.
