Left-hander Joe Thatcher, double-A right-hander Matt Stites and a compensation-round pick are headed to Arizona, according to several media outlets.
The non-waiver trade deadline is 4 p.m. ET.
Kennedy, 28, has a 3-8 record and 5.23 earned-run average in 21 starts this season for the Diamondbacks, who entered play Wednesday in second place in the National League West Division with a 54-52 record, three-and-a-half games behind the Los Angeles Dodgers.
Kennedy, who has lasted seven innings only once in his past 10 starts, has struck out 108 batters in 124 innings but hasn't won in a month. Opposing batters are hitting .268 against him.
Kennedy's most productive season was 2011 when he went 21-4 with a 2.88 ERA and 198 strikeouts in 222 innings.
Thatcher, 31, fills the need for a situation left-hander in the Diamondbacks bullpen, according to general manager Kevin Towers.
He was 3-1 with a 2.10 ERA in 50 appearances out of the Padres' bulllpen this season, walking only four batters over 30 innings pitched while striking out 29.
Stites is 2-2 with a 2.08 ERA and 14 saves in 46 games with double-A San Antonio.