The visitors looked on course for their first win in nine Barclays Premier League games after Michael Owen gave them a fourth-minute lead.
• Carew hails Harewood
But Carew's treble blast after Wilfred Bouma's first ever Villa goal kept Martin O'Neill's side on course for a European spot.
Villa were transformed in the second 45 minutes after barely causing the visitors a problem in the opening 45 minutes.
And their fans taunted the travelling Toon Army with chants of `we want Keegan out' after Carew had become Villa's leading scorer this season with nine goals.
Villa clearly missed the presence of the injured Gabriel Agbonlahor up front in the first half.
But the introduction of Marlon Harewood at half-time made a big difference and the former West Ham forward troubled Newcastle with his physical power.
Newcastle will take some heart from their first-half performance, with James Milner a threat from the right flank against the club he spent a season on loan with.
But Keegan will be concerned with the way his side were overwhelmed for long periods once Villa had stepped up a gear.
Villa started brightly and Shaun Maloney did well to latch onto a flick on from Carew and hold off the challenge of Cacapa before squaring the ball back to Stiliyan Petrov.
But the Bulgarian international was quickly closed down as he lined up a shot and his eventual effort sliced well wide of Shay Given's goal.
Then, after four minutes, Newcastle went ahead courtesy of Owen's second goal in two games.
Damien Duff's initial cross was charged down, but Milner picked up the loose ball and delivered an inviting cross towards the near post.
Owen and Steven Taylor attacked the ball, but it was the England striker who got the crucial touch with a goalbound header which brushed off Villa skipper Gareth Barry before entering the net.
It was a perfect pick-me-up for Owen after being left on the substitutes' bench by new England coach Fabio Capello throughout Wednesday's friendly international with Switzerland.
And it means he has moved above Jimmy Floyd Hasselbaink into seventh place in the list of all-time Premier League goalscorers with 129 to his credit.
Villa tried to retaliate and Barry appealed in vain for a penalty after his fierce shot appeared to strike the hand of Joey Barton.
The deflection wrong-footed Given but also took some of the pace out of the shot and he was able to recover and retrieve the situation before the ball crossed the line.
Newcastle gave away a free-kick in a dangerous area 20 yards out after Taylor tugged at the shirt of Carew, but Maloney curled his effort over the bar and onto the roof of the net.
Newcastle looked the better side and a reverse pass from Milner played in Owen, but he lacked the necessary pace to sprint into the space opening up in front of him and Curtis Davies was able to make the tackle.
Young was being doubled-marked but still got in a cross which found its way to Olof Mellberg at the far post, although he could not keep his shot down.
Villa's best chance of the first half came three minutes before the interval when Given did well to keep out a close-range header from Carew.
O'Neill decided to make a double half-time substitution replacing Mellberg and the ineffective Petrov with Craig Gardner and Harewood.
And the home side turned their fortunes around by striking twice in the first six minutes of the second half through Bouma and Carew.
After 48 minutes Bouma brought the Holte End to its feet with his first goal for Villa after nearly three years at the club.
Maloney won the ball on the left and turned it back into the path of the Dutch international, whose low left-footed shot took a deflection which wrong-footed Given and flew into the corner of the net.
Three minutes later Carew put Villa in front with his seventh goal of the campaign.
Young's inswinging corner was met by the giant Norwegian international, whose glancing header flew into the far corner of the net.
Carew ran away to celebrate with the Villa fans and was promptly booked by referee Lee Mason.
Newcastle were forced to make a substitution, with the injured Given replaced between the posts by Steve Harper.
Villa were now in command and Nicky Butt picked up his 11th booking of the campaign for a foul on Young.
Harper made two superb saves in quick succession to keep out fierce drives from Gardner.
But with 18 minutes left Carew struck for the second time to put Villa 3-1 ahead. Maloney's shot from a Young corner was deflected across goal and there was a scramble before Carew headed home.
Then, with seconds remaining, Carew completed his hat-trick from the penalty spot after Stephen Carr handled.
Carew hails HarewoodHat-trick hero John Carew hailed the introduction of half-time substitute
Marlon Harewood for today's 4-1 Premier League victory over Newcastle.
Villa went into the break behind thanks to Michael Owen's early header.
But Wilfred Bouma's deflected drive and Carew's flicked header turned the game
around within five minutes of the restart before the Norwegian completed his
first treble in English football.
'It wasn't easy to get chances,' Carew said of the first half. 'Marlon came on in the second half and gave us an extra lift with his power
and enthusiasm.
'Then we started to create chances all the time so it was fantastic.'
Carew was also grateful captain Gareth Barry allowed him to take the late
penalty that saw him seal his hat-trick.
'It was fantastic today,' he said on Sky Sports 1.
'The captain's supposed to take the penalties but he said I should take it
and go for the hat-trick and I'm grateful for that.
'Normally I don't smash them but I decided to smash it high to make sure it
went in.'
Villa boss Martin O'Neill was delighted with Carew's display but challenged him
to produce it on a more consistent basis.
'He is absolutely fantastic,' O'Neill said. 'What we want to do is put the averages up. That type of performance every single time would be great.'
O'Neill believes his side's first-half display was a hangover from last
weekend's collapse at Fulham.
'We were very tense. There was probably the feeling of last week when we
hadn't got any momentum going,' he said.
'I thought Newcastle played very well. By half-time they definitely deserved to be in front.
'We had to try and do something about it. 'The players' response was just fantastic and the fans' response.
'Once we got the equaliser early on, we got all the momentum going then and
it was a brilliant second half by us,' O'Neill added on Sky Sports 1.
Aston Villa 4 - 1 Newcastle United
