Pearson left the KC Stadium for a second spell in charge of the Foxes just 18 days ago and it ended up being a miserable day back in East Yorkshire on Saturday after Koren struck a curling effort into the top corner with two minutes of normal time returning.
Pearson's new side had looked like earning a worthy point after Paul Konchesky's neat curling finish five minutes before the break had cancelled out Matty Fryatt's controversial 28th-minute penalty.
That spot-kick was awarded after referee Simon Hooper harshly adjudged Matt Mills to have fouled Aaron Mclean in the area and dished out a second dismissal of the season for the Leicester skipper.
But Koren had the final say and it meant Pearson suffered his first defeat while back in charge at Leicester while the Tigers end a run of two successive defeats in the Championship.
Pearson unsurprisingly received a hostile reception from the home fans but his replacement Nick Barmby was far more welcoming, offering his former manager a handshake and embrace ahead of kick-off.
Unfortunately, the pre-match excitement did not manifest itself into the early match action and a scrappy opening 20 minutes meant a trouble-free start for both goalkeepers.
Hull began to assert themselves, though, and after Fryatt had been unable to turn Koren's drive goalwards the defining moment arrived in the 28th minute.
Mclean broke through on goal only for Mills to make a superb block and as Mclean got up for a second attempt, he appeared to slip over with Mills nowhere near.
However, referee Hooper pointed to the spot and sent Mills off before Fryatt sent Kasper Schmeichel the wrong way for his eighth of the campaign.
It looked as if Leicester would be up against from there on, especially as they had to sacrifice striker Jermaine Beckford, but they equalised 10 minutes later when Konchesky curled a fine right-footed effort into the bottom corner from 20 yards after Peter Gulacsi had saved from David Nugent.
The second half began in atrocious conditions and the hosts started to control proceedings, with Koren unable to make proper contact when presented with a decent chance and the dangerous Cameron Stewart testing Schmeichel from 20 yards.
Schmeichel then came to the rescue for the visitors midway through the second period by producing a fine save, getting down low to turn Andy Dawson's sweet volley round the post.
The Hull onslaught continued as Stewart and Paul McKenna peppered the Leicester goal with efforts as Barmby's side desperately tried to ruin Pearson's return, but Schmeichel was equal to them all until the 88th minute when Koren's superb curling effort went flying into the top corner.