The piano is all of them: string, percussion and keyboard. (and different composers exploited these different aspects, sometimes to extremes).
Can a Xylophone really be a keyboard instrument, if it doesn't really have "keys"? A key [i think] is something you depress to obtain a note [or something you lose, rendering you with an uncomfortable night in the gutter]. If a xylophone is a keyboard instrument, then so would be tubular bells, and maybe even tympani for that matter... the list goes on.Now for a terminology error of my own... aren't all pianos "digital pianos" since you play them with your digits?
A xylophone has its "keys" following the same pattern of the piano keyboard (something that tympani don't). I dind't realise that a keyboard had to be pressed in order to be classified as keyboard (or does it?)
The piano is a string instrument. Any instrument that has strings stretched over a bridge or bridges is a string instrument. How those strings are set into motion is irrelevant.
Whether a piano is a stringed or percussion instrument is dependant on how you play it!
Strings - anything that makes sound by rubbingWoodwinds - anything that makes sound by blowing airPercussion - anything that makes sound by either plucking or striking