Normally, the Qubes GUI virtualization daemon restricts the VM from "owning" the full screen, ensuring that there are always clearly marked decorations drawn by the trusted Window Manager around each of the VMs window.
If one allowed one of the VMs to "own" the full screen, e.g. to show a movie on a full screen, it might not be possible for the user to know if the applications/VM really "released" the full screen, or if it has started emulating the whole desktop and is pretending to be the trusted Window Manager, drawing shapes on the screen that look e.g. like other windows, belonging to other domains (e.g. to trick the user into entering a secret passphrase into a window that looks like belonging to some trusted domain).
However, it is possible to deal with full screen mode in a secure way assuming there are mechanisms that can be used at any time to switch between windows or show the full desktop and that cannot be intercepted by the VM.
The simplest example is the use of Alt+Tab for switching between windows, which is a shortcut handled by dom0.
Other examples such mechanisms are the KDE "Present Windows" and "Desktop Grid" effects, which are similar to Mac's "Expose" effect, and which can be used to immediately detect potential "GUI forgery", as they cannot be intercepted by any of the VM (as the GUID never passes down the key combinations that got consumed by KDE Window Manager), and so the VM cannot emulate those.
Those effects are enabled by default in KDE once Compositing gets enabled in KDE (System Settings -\> Desktop -\> Enable Desktop Effects), which is recommended anyway.
You can always put a window into fullscreen mode in Xfce4 using the trusted window manager by right-clicking on a window's title bar and selecting "Fullscreen" or pressing `alt` + `f11`.
This functionality should still be considered safe, since a VM window still can't voluntarily enter fullscreen mode.
The user must select this option from the trusted window manager in dom0.
To exit fullscreen mode from here, press `alt` + `space` to bring up the title bar menu again, then select "Leave Fullscreen" or simply press `alt` + `f11`.
For StandaloneHVMs, you should set the screen resolution in the qube to that of the host, (or larger), *before* setting fullscreen mode in Xfce4.
As an alternative to the Xfce4 method, you can enable full screen mode for select VMs by creating the following entry in the `/etc/qubes/guid.conf` file in dom0:
**Note:** There should be only one `VM: {}` block in the file (or you will [get into problems](https://groups.google.com/d/msg/qubes-users/-Yf9yNvTsVI/xXsEm8y2lrYJ)).
