Dewan Bahadur / Diwan Bahadur was a title of honor awarded during British Raj. It was awarded to individuals who had performed great service to the nation. It is the equivalent of an OBE. The title was accompanied by a medal called a Title Badge. Dewan literally means Prime Minister in Indian context and Bahadur means brave.
This title was above Rao Bahadur title and usually people with Rao Bahadur were elevated to status of Dewan Bahadur.
Further, the Prime Ministers of Indian Princely States were known as Dewan/Diwan. They were also given or promoted directly to the title of Dewan Bahadur by British authorities on being appointed as Dewan, to suit their post.
The Dewan Bahadur and other similar titles issued during British Raj were disestablished in 1947 upon independence of India.