The PDW is a submachine gun, manufactured by Heckler & Koch, that is compact, lightweight, low recoil, and fast firing. Strangely, this does not seem to be a very good weapon for personal defense because it is still an SMG: larger than a handgun, and is a wee bit overpowered for neutralizing your common criminal.
The standard PDW and the newer MP7-PDW could very well outclass the MP5 in the police force \ SMG field, except for its smaller magazine capacity.
Two weapons currently fit such a description, FN Herstal's FNP90 (5.7x28mm)and HK's MP7 (4.6x30mm). Both of these weapons are marketed as being perfect for support staff and pilots where a full size rifle would be too large to be practical