Since reaching the majors at 27 -- and finishing fourth in Rookie of the Year balloting -- Aviles has been up and down, but seems to have settled in a pretty good hitter for a guy who can play all four infield positions (or probably could, if asked to play first base). With the Red Sox, he'll serve as backup for fragile shortstops Marco Scutaro and Jed Lowrie, and (very) occasionally spell Kevin Youkilis and Dustin Pedroia.
Volz is a 23-year-old Class-A relief pitcher. He's big and has gotten a lot of strikeouts this season, but according to this report doesn't throw particularly hard. He's organizational depth, with a shot at pitching for the Royals someday, maybe.
Navarro, also 23, has been impressive enough to play some games with the Red Sox last season, and this season. But he's terribly slow, reportedly because he doesn't watch his weight. He doesn't figure to play regularly for the Royals, who have Alcides Escobar at shortstop and Chris Getz at second base, with second-base prospect Johnny Giovatella tearing up Triple-A this summer.
Rather, Navarro figures to fill Aviles' utility role, but younger and cheaper.