Crowned Bundesliga champions in mid-April, Leverkusen were made to fight hard against their second-division opponents, playing with a man down after Odilon Kossounou’s second yellow late in the first half.
Xhaka’s 16th-minute goal, a beautiful long-range strike with his left foot, made the difference as Leverkusen bounced back from Wednesday’s Europa League final loss to Atalanta -- which ended both their season-long unbeaten run at 51 games and their hopes of a treble.
“To win like that at the end, we showed our team spirit. We fought hard, we did it for everyone, the fans, the club,“ Xabi Alonso told Germany’s ARD.
“To achieve the double. It’s been a great season”.
Alonso won praise from his opposing coach Friedhelm Funkel, 70, who was managing his last game before retiring.
“To go through the Bundesliga season unbeaten, it’s madness. That he could flick that switch in his first year, is incredible.
“I think he’ll win many, many titles.”
Underdogs Kaiserslautern
Kaiserslautern, four-time German champions stranded in the lower leagues since 2012, were bidding to become just the second club from outside the top division to lift the Cup after Hannover in 1992.
Narrowly avoiding relegation to the third league this season, Kaiserslautern only faced one top-division club on the way to the final -- the since-relegated Cologne.
Kossounou, who was left out of Wednesday’s loss in Dublin, returned to the starting line-up and picked up a yellow card after just two minutes for a robust challenge on Kenny Prince Redondo.
Leverkusen then took the lead after 16 minutes, long-range expert Xhaka blasting in from well outside the penalty area.
Kaiserslautern almost struck a blow moments after the red card when Tobias Raschl lashed a shot inches wide of the post.
Kossounou’s early yellow proved crucial when the Ivorian was sent from the pitch after a second booking in the 44th minute.
Alonso, who sat with his head in his hands after Kossounou’s dismissal, made two changes at half-time, hooking striker Patrik Schick and winger Jonas Hofmann in favour of defensive reinforcements.
Driven on by a passionate set of travelling fans who unveiled a giant Red Devil, the club’s mascot, just before kick-off, Kaiserslautern smelt blood but lacked the quality to break Leverkusen down.
Leverkusen’s incredible season was built on front-foot football but Alonso’s side were content to sit back and defend in the second half before attacking on the counter.
The experienced ex-player Alonso also tried to take the chaos out of the game, at one point stopping his own side’s counter attack by grabbing Florian Wirtz with both arms to halt a throw-in.
Alonso said he tried to “calm down” his attacking midfielder, adding “we want control”.
“He wants to be quick, but I wanted him to stay calm and control the tempo.”
Leverkusen held on to win another piece of silverware this season, doubling the trophy tally in the club’s entire history, after winning the German Cup in 1993 and the 1988 UEFA Cup.
The final was the first not to feature one of Bayern Munich, Borussia Dortmund or RB Leipzig since 2012 -- who boast a combined 27 German Cups.
