I tend to believe there is an issue between XFCE and the dedicated Nvidia driver. I spent a lot of time trying to make it working but the driver is not loaded and, yes, I followed the steps available on here, on Qubes webpage, Fedora forums etc…
Nothing worked 
Based on the latest kernel version(s) someone is working to improve the coverage of Nvidia graphic cards with the nouveau driver but still not there yet.
Check my thread here