There is no difference between Kenpo and Kempo, they are different spellings of the same martial art.

The Japanese kanji characters for kenpo and kempo are the same, yet when translated to English, the N can also be an M.

Kenpo places equal emphasis on the use of hands and feet and uses similar fighting techniques to other Okinawan fighting styles. Kenpo also practises Kata or forms like other martial arts. Whereas most Karate styles use white Gis (uniforms) throughout, a visual characteristic of Kenpo is its use of black Gi’s for higher grades and even the mixing of black Gi tops with white pants and vice versa.
Kenpo Karate is a complete fighting system that is particularly popular in the United States.

Like most Okinawan fighting arts, Kenpo Karate can trace its roots back to the Shaolin monks of China. Master “To-De” Sakugawa (1733-1815) from Shuri, the ancient capital of Okinawa, travelled to China in the 18th century to train with the Chuan Fa masters (Chuan Fa is what Chinese Kung Fu was called at that time).

On his return to Okinawa he developed what became known as Shuri Te, from which Kenpo was later born. In contrast, the Okinawan martial arts developed in Naha, the modern-day capital of Okinawa, first became known as Naha-Te, and developed later on into Goju-Ryu Karate).

In 1916 a young Hawaiian named James Mitose travelled to Kyushu in Japan where he learnt Kosho Ryu Kempo. He later returned to Hawaii where he taught William Chow, who further developed the art.  To differentiate his system from that of Mitose, William Chow called his school Kenpo Karate. As a visual break from the traditional Japanese and Okinawan Karate styles, Mitose and Chow introduced the wearing of black Gi’s for higher ranks, to indicate that Kenpo was a different and more of a “war art” than the increasingly sports-oriented, white-Gi-wearing Karate styles.

Ed Parker was a student of William Chow and is considered the father of American Kenpo, as he had the greatest modern day influence on the spread of Kenpo around the world. He opened the first ever university campus martial arts school in Utah USA in 1954, at the age of only 23 and later became a tournament promoter. At one of his early tournaments, Bruce Lee first came into the view of the general public.
Ed was also active as a movie actor, stunt coordinator, author and instructor to many famous Hollywood actors. ... more information

Shorinji Kempo  - for the sake of accuracy particularly in this picture, the kneeling, robed kenshi defending against two attackers are wearing garments unique to Shorinji Kempo - which is a distinct martial art founded in 1947 by Doshin So.  It is not Okinawan and is unrelated to the Kenpo.
The word Kenpo or Kempo, when written alone, is generic (it translates as ‘boxing’ or ‘fist way’). As a result the word forms part of the name of many different martial arts. By contrast, the term ‘Shorinji Kempo’ is registered as a trade mark. 
More information about Shorinji Kempo can be found via the websites www.bskf.org  or www.shorinjikempo.or.jp/wsko