Putting aside that the use of 'thx' is bad, and it isn't really a word. Would 'thx' be considered a contraction or abbreviation?
Contraction: a shortened version of the written and spoken forms of a word, syllable, or word group, created by omission of internal letters (actually, sounds).
The page also states that a contraction "shouldn't be confused with an abbreviation or acronym. My logic says that it would be an abbreviation since a contraction would also change the phonetic version of the word while an abbreviation does not.