If you receive a poor picture or incorrect system behavior when switching a guest operating system to fullscreen mode, this can be fixed by configuring your primary OS settings.
Windows primary OS:
When guest OS is running in fullscreen mode, physical display is switched to guest OS resolution with the highest possible frequency. If your physical videocard or physical monitor do not support the resolution or frequency requested by the guest OS, substitute video mode that matches best is set instead. In some cases substitute mode may not work, or you are warned of an inability to switch to fullscreen mode. To fix the situation, correct primary OS display settings:
Linux primary OS:
When a guest OS is running in fullscreen mode, the physical display is switched to guest OS resolution with the highest possible frequency. If X Window System is not configured for the mode requested by guest OS, substitute mode that matches best is set instead. The situation is worse if the physical monitor does not support the requested mode while it is configured in X Window System. Parallels Workstation can not know that the monitor does not support the requested mode and will try to switch to it. This may lead to poor picture quality in fullscreen mode.
If display picture in fullscreen mode is unusable: