The clue is in the word "Trailer", which in railway terminology indicates an un-powered vehicle.

Trains with a DVT operate on the same principle as the "push-pull" trains dating back to around 1900, initially for short local branch-line trains. The (steam) locomotive remained at one end of the train, and the carriage at the other end had a compartment for the driver, who could operate the regulator and brake, leaving the fireman on the engine to tend the fire, injectors, reverser etc. The system was (I believe) pioneered in the UK, and taken up in many other countries, France especially.

By the 1980s it was demonstrated that a locomotive could safely propel high-speed full-length express passenger trains. The "Gatwick Express" service between London Victoria and Gatwick Airport was one of the earliest, and others followed, using modified passenger carriages as the driving trailer. The modern Driving Van Trailer is a development of the principle, having a streamlined driving end more suitable to high speed running, with the remainder of the vehicle being given over to the guard's and luggage accommodation.

The main reason for push-pull operation is that it avoids having to attach a fresh locomotive to the rear of the train at terminals (and uncoupling the 'incoming' locomotive), or having to provide the necessary track-work to enable the incoming locomotive to 'run round' the train to the other end.

A DVT operates the locomotive at the other end of the train. It isn't powered in its own right. Some, though, are being fitted with a diesel generator to provide air and electricity for the train when the locomotive isn't operating, such as in a terminus.

All DVTs [ 821xx ex WCML; 822xx currently ECML; 823xx currently Chiltern / ATW and 97xx currently Anglia] are unpowered as their prime purpose is to provide driving controls at the front of the train using the power from the traction unit at the rear. This configuration was adopted to save locomotives running round trains at terminals such as Kings Cross; Euston and Liverpool St where there are numerous train movements and the reduction of running round movements helps increase the number of revenue-earning trains that can be operated.

Driving Van Trailer