Pacheco's home run came during a five-run fifth and he also drove in a run with a ground out in the second.
Colorado took a 10-4 led through six innings and fought off a Giants' rally that ran out of steam after Buster Posey hit a solo shot in the eighth.
Pacheco, in his third season with the Rockies, hit his first home run of the year and his eighth in 179 major league games.
Carlos Gonzalez also had a two-run single for the Rockies in the third and contributed a solo homer in the sixth.
Pablo Sandoval and Posey had two RBI each for the Giants, who outhit the Rockies 15-10.
Madison Bumgarner (4-2) was roughed up for seven runs on eight hits in 4 2/3 to take the loss.