The Catalans fell a goal behind in the ninth minute when Robinho took advantage of some sloppy defending to slot home, before Lionel Messi restored parity halfway through the first half.
Nevertheless, Barcelona failed to add a second and eventually had to settle for one point.
"The only thing missing was the second goal. It’s a shame because we created the chances to win the game. We were lacking something in front of goal," Xavi told reporters. "We dominated with the exception of the first five minutes. We created a few chances and provided a good account of ourselves.
"It's never easy to play at San Siro, so we can be glad about the point even if we wanted three – I think we deserved them. We are still unbeaten and we are playing well. There was good intensity against Milan and we can be satisfied for the level of our performance."
Barcelona tops its section with seven points from three games, holding a two-point lead over Milan.