These related words have a common Latin base, a term meaning "to remain standing." A restive person or animal is unable to remain at rest (that is, quiet) because he cannot bear restraint or control and becomes impatient with delay: "When tied to a hitching post, the horse became restive." Restless is a commonly used word applied more to unquiet and unease of the mind or emotions than of the body and usually suggesting the absence of actual restraint: "This restless patient has grown restive under hospital regulations." Restive, unlike restless, also implies refusing to move, being balky or recalcitrant. A horse chafing under a bit and refusing to obey its rider's commands would be termed restive, not restless.
Dictionary of problem words and expressions. Harry Shaw. 1975.