Villa were two goals ahead after 36 minutes but were unable to hold on.
They were left to regret half a dozen clear chances in a first half which they dominated from the start.
Striker Peter Crouch, recalled in place of the suspended Dion Dublin, endured a nightmare match, missing four clear chances to have got off the scoring mark for the season.
In addition Darius Vassell hit a post after going around goalkeeper Antti Niemi.
Southampton went into the match with a weakened side and it showed in a poor first-half display and until Davies equalised, the second-half improvement did not look like being enough.
Lee Hendrie put Villa ahead after half an hour with a perfectly struck right-foot shot which gave Niemi no chance.
Villa were further in front after 36 minutes when Norwegian defender Claus Lundekvam made a rare error in under-hitting his back-pass to enable Vassell to run clear to score.
But Southampton seized on Villa's inability to take further chances when James Beattie notched his 21st goal of the season with a low shot after Ronny Johnsen had blundered in passing the ball to him.
Beattie also hit a post and Saints were much improved after the break.
Even so, they had to wait until the last minute of normal time when Johnsen failed to clear a long ball out of defence and Davies struck from 15 yards.