Kenna notched the vital 67th minute winning goal for the Midlands club and a minute later was laid flat on his back by a flying elbow from David Prutton.
Prutton was immediately shown the red card by referee Steve Bennett, while Kenna had to leave the field for attention to a cut above his left eye.
At the start Southampton made light of Gordon Strachan's midweek bombshell to leave the Saints at the end of the season. Instead, they were quickly firing bullets aimed at the downfall of Birmingham.
With James Beattie showing some impressive form in the early stages Birmingham's makeshift defence was stretched to the limit.
Birmingham boss, Steve Bruce, paraded the team which finished the game against Spurs the previous week and initially it appeared not to have been the right move.
Irishman Kenna was again in the problem left-back position and it was from a corner on the right that Southampton took their shock sixth minute lead.
Jason Dodd's cross deep into Birmingham's defence found Rory Delap. He fired in an accurate and powerful shot which goalkeeper Maik Taylor could only parry.
Brett Ormerod was on hand to score his fourth and possibly his easiest goal of the season.
It proved a wake-up call to Birmingham who sadly missed the suspended Robbie Savage.
But David Dunn came of age in Birmingham's colours and produced one of his best performances.
As a result Birmingham suddenly became more dangerous, but it was from another right-wing corner which produced their equaliser in the 17th minute.
Bryan Hughes floated over a cross which was nudged on by Kenny Cunningham for Stephen Clemence to head in his third goal of the season following a slight deflection off Fitz Hall.
It took Strachan 25 minutes before he left his seat in the front of the stand to begin a non-stop barrage of instructions to his players.
He had every reason to be so demonstrative as Southampton were run ragged for a spell with Dunn enjoying his best game since his move from Blackburn last summer.
But Strachan could have no complaints with the combined goal-line heroics of Antti Niemi and Anders Svensson as they combined to keep out a Darren Purse header just before the break from another Hughes' corner.
But Niemi had little chance with Kenna's 67th minute winner.
The Irishman a strong run pushing the ball wide to Clinton Morrison who provided a low cross into the middle which the determined Kenna ran on to score with a shot on the run.
In the next minute Kenna was flattened by Prutton with an elbow above his left eye. He had to leave the field for attention, but returned to earn the man of the match honours.
Morrison threatened a third goal for Birmingham but with Niemi out of position his cross clipped the crossbar.
Rupert Lowe, Southampton's chairman, and Gordon Strachan gave contrasting views upon why the little Scot is to leave the Premiership club at the end of the season.
Lowe said: "We have made perfectly clear with our statement where we stand.
"Gordon has a problem with his hip which he made known at the press conference.
"He has made some complimentary remarks about the club and how he has enjoyed working at Southampton. So those people who are reading more into the situation are way off target.
"But Southampton fans can rest assured that we will do what is right for the club."Lowe, when pushed, admitted that it was an on-going situation and there was a possibility that Strachan might not see out the end of his contract.
Strachan, however, added to the confusion about his decision to leave Southampton.
Strachan said: "My decision was made two and half years ago. It is not for health reasons. I do have a small problem with my hip but it's as simple as that.
"I've been working for 31 years and my family and myself feel it is time to stop and smell the roses.
"I informed the club in October and there is no other reason for my decision. I'm now off for a three day managers' coaching course at the Forest of Arden."
Birmingham boss, Steve Bruce, thought it was probably his side's best result in 18 months to come back from an early goal and win.
He was full of praise for the Republic of Ireland international defender, Jeff Kenna, who scored the winning goal.
He said: "You expect this kind of performance from Kenna He has been around the top flight for a long time.
"I just wish he was 26 or 27, but his future at Birmingham next season is assured."