According to a spanish book about names, Ximena is an ancient form for Jimena, because in the Middle Ages, the sound [x] was written with an X. In the XVIII century, the spanish ortograph changed and the sound [x] started to be written with a J or a G.
Jimena (and Jimeno (m)) is an ancient form for Simeón. It comes from Navarra.
By the way, a famous Jimena was the wife of El Cid. When her husband died, in 1099, she became Queen of Valencia until her death, in 1101.