In 1930 the Motion Picture Association of America came up with Motion Picture Production Code (then known as the Hays Code after Will H. Hays), a code of general principles, and gave a seal of approval to those pictures which adhered to the principles. Thus there were effectively two categories, approved and non-approved. This lasted until 1968, two years after Jack Valenti became president of the MPAA.

Valenti wanted to create three different rating categories, G, for general audiences, M, for mature audiences (parental guidance suggested but not required), and R, for restricted (under 16 not permitted without a parent). However "NATO urged the creation of an adults only category, fearful of possible legal redress under state or local law". The solution Valenti came up with was to introduce a category X, but not to trademark the X rating (all the other ratings were trademarked). Thus, any film could call itself "X rated" without getting permission from the MPAA. The MPAA did assign some films the X rating, but most films which used the rating, especially in the porn industry, just applied it to themselves without asking for or receiving the permission of the MPAA.

XX and XXX were never categories introduced by the MPAA. They were marketing gimmicks invented by the pornography industry. Like the X rating they were never trademarked, so anyone could and can use them to mean absolutely anything. So in essence there is no difference between X-rated and XXX-rated, except that it's possible (though highly unlikely) that the X-rated film had received that rating from the MPAA.

As for XX-rated, what would be the purpose of calling your movie XX-rated? You'd apply the X rating if you wanted to go with the original, and the XXX rating if you wanted to indicate that you were the naughtiest of the naughty. There's no marketing gimmick in being "naughtier than most but less naughty than others", is there?

Why X, XX, and XXX? One possible explanation was that the alcohol content of beer (another naughty indulgence) was designated as X, XX, and XXX.

In the United States, no "X" ratings are official anymore. XX and XXX were never official.

The X-rating was part of the MPAA rating system from its inception in 1968. Once "X" was latched onto by the porn industry and used for marketing (XX and XXX being used to denote even dirtier material than the MPAA's X-rating), X-rating became NC-17 in 1990.

Film scholar William Rotsler wrote, "The XXX-rating means hard-core, the XX-rating is for simulation, and an X-rating is for comparatively cool films."

Based on some research I just made, it appears that officially the XXX rating does not exist.

You will find the answer to your question here:
http://www.pbs.org/wgbh/pages/frontline/shows/porn/interviews/mcalpine.html

Find this excerpt:
"Tell me what X is, XX and XXX. What is it, content-wise?

There is no official MPA rating. They call Adult NC-17, and I'm not even quite sure what NC-17 stands for. But the XXX rating that you see on the old adult theaters was something that was gimmicked up by the exhibitor, who was trying to make the movie look bigger than it was. "Gee, come here and you're going to see Deep Throat and all of these things." There is no XXX rating."

See official movie ratings here:
http://en.wikipedia.org/wiki/Motion_Picture_Association_of_America_film_rating_system