From Ronald D. Cohen's review of Anthony Harkin's "Hillbilly: A Cultural History of An American Icon" (New York: Oxford University Press, 2003):

Stereotypes, whether positive or negative, of southern mountain dwellers and rural folk more broadly, date from the colonial era. William Byrd painted a generally negative picture of rural southerners in his diary entries in 1728: "unhealthy, slovenly, and utterly averse to work" (p. 15). By the end of the nineteenth century the image of the lazy southern (but not northern) backwoodsman became complicated by the addition of the menacing yokel, fueled by the combination of family fueds and moonshining. There also developed just the opposite image, of the virtues of the racial (Anglo-Saxon) and religious (Protestant) purity of the mountain folk. The term hillbilly emerged slowly in the twentieth century, along with a plethora of films that combined humorous and deadly characters. When a market developed for rural recorded music in the 1920s, hillbilly became the commercial designation of southern string bands and performers; radio programs, such as the WSM Grand Ole Opry, simultaneously emerged, with the yokel stereotype predominating into the 1950s. 2

Harkins next describes the cartoon images that emerged in 1934, particularly Paul Webb's series in Esquire magazine, The Mountain Boys, simultaneously with Billy DeBeck's Barney Google comic strip introducing the new character Snuffy Smith, and Al Capp's Li'l Abner. While not quite the same, "The works of Webb, DeBeck, and Capp changed the dominant definition of the mountaineer from genuine threat and danger to harmless, if aberrant, comical transgression.... At the same time, their work retained the ambiguities inherent in this cultural construction by unifying derogatory conceptions of backwardsness, ignorance, and savagery with positive ideas of ruggedness, independence, and devotion to family and home" (pp. 139-140). Hollywood movies in the thirties and forties followed suit, usually depicting "a comically backward yokel" (p. 141) such as the Ma and Pa Kettle series. Subsequent television programs continued the trend, such as The Real McCoys, The Andy Griffith Show, and The Beverly Hillbillies. Many viewers were offended by such blatant, comical stereotypes, yet such shows retained their popularity, along with lucrative merchandising gimmicks. But by the late 1960s "the hillbilly vogue" had basically faded away, except for the popular, family-oriented Waltons, a show which lasted through the seventies.

Source: History of Education Quarterly Vol. 44 No. 3 (Fall 2004)
http://www.historycooperative.org/journals/heq/44.3/br_13.html