There is not a great deal of difference between the two. And from the OED definitions (below) it is difficult to identify any at all. It is interesting that the entries on joyous refer the user to joyful but not vice-versa.

Joyous seems to me a slightly dated word, and has, I believe been used more in a religious sense. It has a ring of the James I Bible about it. My own inclination would be to reserve joyous for more sober enjoyment. Whilst a booze-up at the pub might be said to be a joyful occasion, when describing a reunion of long-lost relatives joyous would seem more apt. I would not be inclined to use joyous to describe a person - joyful seems the better word.

The NGrams are interesting, but tell us little, other than that both words have been in long-term decline.

OED Definitions

Joyful

Of persons, their feelings, etc.: Full of joy; having and showing a lively sense of pleasure or satisfaction; elated with gladness, delighted.

Of action, speech, looks, etc.: Expressing or manifesting joy; indicative of gladness.

Joyous

Having a joyful nature or mood; full of glad feeling; blithe, gladsome, buoyant; also, expressive of, or characterized by, joy; = joyful adj. 1, 2.

Of things, events, etc.: Inspiring or productive of joy; gladdening, cheerful; = joyful adj. 3.