Aviles was twice a .300-plus hitter for the Royals, but earlier this season was optioned to Triple-A Omaha because of a fall-off in his hitting and fielding.
Navarro, who played against the Royals in this week's four-game series at Boston, played in 16 games for the Red Sox and batted .216 (8-for-37). In the series against the Royals he was 4-for-12 with a double and a RBI while playing three different positions.
"We like the fact that he's just 23 years old, can play all three infield positions at third, short and second and can play some outfield," Royals manager Ned Yost said. "We like the way he can swing the bat. He's still a young hitter and needs a little more plate discipline, but he's got a great upside."
From San Pedro de Macoris, Dominican Republic, Navarro was limited by injury to 34 games this year with Triple-A Pawtucket and batted .258. In 2010, Navarro hit .275 with 11 homers and 61 RBIs in 104 games combined for Pawtucket and Double-A Portland.
"Navarro is somebody that we control for six more years and he's just 23 years old," general manager Dayton Moore said. "He can play all three infield positions, he's better at short and third and a right-handed bat, which fits what we're trying to do in the corners. It gives us more depth in the organization on the left side of the infield."
Navarro was expected to join the Royals after their game against the Indians on Saturday night was well under way.
Volz, also 23, is a right-handed reliever who was 2-3 with two saves and a 3.33 ERA in 31 games for Class A Salem. He had 56 strikeouts in 51 1/3 innings and held opponents to a .222 average.
A ninth-round Draft choice by the Red Sox in 2009 from Baylor University, Volz will report to the Royals' Class A club at Wilmington.
Last year, Volz was a starter for Class A Greenville and was 6-5 with 3.71 ERA in 26 games. He played for Team USA in 2008.
"We've seen him this year and like his arm and his ability to pitch," Moore said. "He's got a good breaking ball."
Aviles, 30, was hitting .222 with five homers and 31 RBIs in 53 games this year for the Royals.
"I was pretty surprised. I didn't think anything was in the works. I didn't know," Aviles said after joining the Red Sox in Chicago. "It's a good surprise, in that case, to come to a team that's a perennial winning team and always has a chance to be in the playoffs and win a World Series, and I'm fortunate enough to be part of that right now."
After five-plus seasons in the Minors, Aviles burst onto the Royals' scene in 2008 and became the Royals' regular shortstop in early June. He batted .325 with 10 homers and 51 RBIs in 102 games and was voted the Royals Player of the Year.
In 2009, however, he had trouble with his throwing arm and underwent reconstructive surgery on his right elbow on July 7 that year. He played just 36 games and batted .183.
Last year, after a brief early-season stay with Omaha, Aviles became the Royals' primary second baseman ahead of Chris Getz and finished with a .304 average, eight homers and 32 RBIs in 110 games.
This year, Aviles began with the season sharing third base with Wilson Betemit, but was sent to the Minors on June 9 when Mike Moustakas was called up from Omaha.
"He kind of got bypassed," Yost said. "We got [Alcides] Escobar in the trade and that tied up short. Our plan all along was to get Moose up here and that tied up third. And he was struggling defensively at second with the ability to turn two at times."
Aviles said he was quite willing to take a backup role with the first-place Red Sox.
"I'm basically in a utility role and do what I've got to do and be ready to play every day. When people need a day off, I'll jump in there," he said. "However the manager [Terry Francona] wants to use me, that's perfectly fine. I'm just happy to be here and happy to contribute any way I can."
Aviles played shortstop at Omaha because hot-hitting Johnny Giavotella was entrenched at second base. Getz held that position for Kansas City.
Giavotella is the Storm Chasers' leading hitter with a .334 average and figures to compete for the Royals' second-base job next year.
Aviles was recalled on July 20 after posting a .307 average for Omaha with nine homers and 25 RBIs in 35 games. That was the day the Royals traded Betemit to the Detroit Tigers for two Minor Leaguers.
Those are the only two deals the Royals have made prior to the Trade Deadline at 3 p.m. CT on Sunday. After the Aviles trade, Moore said that the Royals had no other deals imminent.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.