As cultural background I'd add that this goes back long before electronic communications. We used to write them on postcards and letters in the 1950s. (My aunt taught that to me. She also taught me that an upside-down postage stamp is a secret way to tell someone that you love them!) In fact, Google Books is showing me a 1906 court case, http://tinyurl.com/qgyz3lc , about a message that said "1000 million X O," and that "Prosecutrix... testified that defendant told her that when he wrote X and O he meant hugs and kisses."

x = kiss o = hug So it's supposed to be like they're sending you their love through hugs and kisses. (Most likely in a friendly way.) That's the only thing I figure that they would mean.

It means kisses (x) and Hugs (o)