Joe Corona added a third on a long-range effort to ensure victory and avenge defeat in the teams' last meeting during group play at the MLS is Back Tournament in July.
Pavon produced his goal on a sensational solo effort, his fourth tally in his past five matches, the same span in which Los Angeles has been without star striker Javier "Chicharito" Hernandez due to a calf strain.
The Galaxy have now won three straight matches and earned 10 points overall during Chicharito's absence.
Substitute Diego Valeri scored his 80th career MLS goal in second-half stoppage time for Portland. Felipe Mora also scored his second goal in as many games to bring Portland briefly within 2-1 earlier in the second half.
Alvarez put the Galaxy ahead in the 15th minute after combining with Julian Araujo and Sebastian Lletget down the right. Eventually Lletget got to the end line and cut a cross back to Alvarez, who curled a left-footed shot beyond Attinella's dive from near the penalty spot.
It was 2-0 five minutes after the break. Pavon took a diagonal ball from inside his own half, and with his first touch evaded defender Pablo Bonilla to get down the left flank. Then he cut inside on Julio Cascante into the penalty area and fired a low shot through traffic to beat Attinella.
Mora answered in the 67th minute. Galaxy goalkeeper David Bingham swatted away a deflected cross but failed to push the ball out of danger. Mora reached the rebound and fired a low, hard finish between Bingham and the left post.
But Los Angeles led by two again only four minutes later, when Corona reached a half-clearance and smashed a 20-yard strike into the top left corner. And despite Valeri's late strike, the Timbers could not pull off a miracle late rally.