The Hovhaness is indeed for xylophone and orchestra, although it has commonly been performed on marimba. (Xylophone works better in the orchestra version!) The piano reduction is just that, a reduction of the orchestra score. Since there are extended interludes with just orchestral percussion accompaniment, something is lost in the piano version and some have chosen to add percussion to the piano. The percussion is cued in the piano reduction.
There are a couple of OK recordings out there, but even the superior original Hiraoka LP recording has an unfortunate cut of the lyrical 2nd theme.
As a 1965-era two-mallet xylophone concerto, it is difficult to compare with a more recent four-mallet marimba concerto. My votes for all-time best marimba concerti are still with the Richard Rodney Bennett, the Peter Klatzow, and the Tomas Svoboda. Also check out the recent Rihards Dubra and the Eric Ewazen concerti.
William Moersch