Nothing, just the letters. Yamaha in all is wisdom decided to rebrand the XS a TX in 73 when they introduced the TX750 and TX500 keeping them all the same as TX's. When the 750 bombed they removed it from the market in 75, keeping the 500 for another 3 years and rebranding the 650 and 500 as XS's to remove them from the stigma of the TX brand
74 TX650A and 75 XS650B are identical except for the paint job
TX750 did have the problems solved by the time the 74 model had come out but it was to late as the public had given it the old thumb's down