NEWCASTLE 3 SOUTHAMPTON 0
Dubravka, Yedlin, Lascelles, Lejeune, Dummett, Ritchie, Shelvey (Merino 83), Diamé, Kenedy (Atsu 78), Pérez, Gayle (Joselu 69)
Subs: Clark, Murphy, Manquillo, Darlow, Atsu
Everything was positive on Saturday for Newcastle as they climbed to 13th and five points clear of danger.
And the good energy which was coursing through a sold-out St James' Park was provided by Chelsea loanee Kenedy, who also overcame a midweek fitness scare to feature.
There is a fearless naivety about the 22-year-old - perhaps that explains the offensive social-media posts which saw him sent home from Chelsea's tour of China last year - but his daring is exactly what Benitez's side had been missing until his January arrival, since when they have lost only one of six.
Kenedy's opener defied physics, somehow chesting Shelvey's dropping pass back up in the air and spinning before volleying home. His second was far easier, turning in from a few yards out, but he had showed ambition to make the burst into the goalmouth and got his reward, as he did when St James' rose to their feet when he departed just before Shelvey.
'It's more than his energy and pace,' said Benitez when considering the difference Kenedy has made. 'He is a player who is comfortable on the ball - his crosses, his set-pieces, he is somebody who is different and he is a player with confidence right now. He does not give the ball away easily and he gives us more control.
'I have no idea if there is a chance we can keep him after the end of the season, I think at the moment we have to make sure we enjoy him while he is here.'
While Newcastle's supporters are doing that, Southampton's are not enjoying anything right now.
After one win in 17 the players were accused of 'giving up' by manager Mauricio Pellegrino, who was booed as he got on the team coach afterwards. Some of those fans were wearing Ali G and Super Mario costumes, but there is nothing funny or heroic about their side. They are going down without a fight.
The opposite is true of a spirited Newcastle, whose players are fighting for fitness and survival. They, unlike Southampton, should be just fine.
Southampton's best chance fell to substitute Josh Sims, but his 20-yard strike was well saved by Martin Dubravka.
This result, which came after Newcastle beat Manchester United in their previous home game, again highlights the great job Rafael Benitez is doing in trying circumstances.
With owner Mike Ashley wanting to sell the club and a proposed takeover falling through in January, Benitez was left without money to sign any players in the latest transfer window.
Instead, the ex-Liverpool, Chelsea and Real Madrid boss took three players on loan - impressive goalkeeper Martin Dubravka from Sparta Prague, Brazil Under-23 international Kenedy from Chelsea, and striker Islam Slimani from Leicester, who is yet to play for Newcastle because of injury.
On Saturday he was excellent. With barely a minute gone, Shelvey played the ball over the top and Kenedy showed great skill to jump, take the ball on his chest, spin past defender Cedric Soares and give Newcastle the lead.
His second goal came less than 30 seconds after Southampton had taken a corner. Saints midfielder Mario Lemina failed to connect with a shot, Perez led a counter-attack and played it into Gayle, whose first-time unselfish pass gave Kenedy a tap-in.
Newcastle's third goal was helped by more poor Saints defending as an unmarked Shelvey passed to Ritchie, who had time and space to guide a low shot into the bottom corner.
Southampton are playing in their sixth consecutive season in the Premier League, and have not finished lower than 14th in any of the previous five campaigns.
But they must improve dramatically on this performance if they are going to have another year in the top flight.
They had 63% of the ball, but did little with it and did not register a shot on target in the first half.
They were not helped by some terrible defending as they gave Shelvey far too much space to play in passes for the first and third goals, and to concede less than 30 seconds after taking a corner is not good enough in the top flight.
Saints midfielder Pierre-Emile Hojbjerg was unhappy with the performance of his side and said: "There is no chance of us going out like this again next week. We are much better than this, we have no excuses."
The result will also put the future of manager Mauricio Pellegrino, who played under Benitez at Liverpool and Valencia, in doubt.
Pellegrino was appointed in the summer, when an eighth-placed finish and League Cup final appearance were deemed not enough for former boss Claude Puel to keep his job.
This result means Southampton have won just once in their past 17 league games, but what may be of greater concern was the manner of their defeat and their capitulation.
The Saints have eight games left to secure their survival, including tricky-looking fixtures against Arsenal, Chelsea, Leicester, Everton and Manchester City, although their fixtures also include matches against bottom-half teams Swansea, West Ham and Bournemouth.