Glossolalia: In general coming from the Greek, "glossolalia" can refer to 1) speaking in either a variety of different languages or 2) speaking in incomprehensible sounds/gibberish/non-real languages.
Specifically, "glossolalia" in common English properly refers to 1) making noises, including bodily noises, sighs, moans, etc., or 2) speaking in gibberish/non-real languages
Xenolalia: "xenolalia" refers to speaking in many real languages. In apostolic times, this would refer to having an infused gift/learning of a language/number of languages (see Acts 2). In current times, "xenolalia" refers to the ability to learn foreign languages; some individuals learn non-native languges with ease while others struggle/find it impossible. Those who find it relatively easy/accessible are said to have the gift of "xenolalia." It can be considered a subcategory of the general term "glossolalia," but for specific usage, "glossolalia" and "xenolalia" denote two different things.
St. Paul did not have this distinction, so interpretation of the passages in which he refers to speaking in tongues must use the surrounding context, both textual and Christian, to determine which type of tongues he speaks about.
Gingrich, F. Wilbur. Shorter Lexicon of the Greek New Testament. Chicago: University of Chicago Press, 1983
"Glossolalia" and "Tongues, Gift of" in Catholic Biblical Encyclopedia: Old and New Testaments, John E. Steinmueller and Kathryn Sullivan, (New York: Joseph Wagner, 1956), 258-9 & 635-6.