It was a stunning blow to Villa, and more frustration was to follow when Agbonlahor's header looked destined to bring the equaliser but Bacary Sagna produced a breathtaking overhead kick to clear the ball off his goalline.
And, after the break, Arsenal delivered a hammer blow when Diaby broke from inside his own half, laid the ball off to Emmanuel Eboue, continued his run and smashed the ball in after collecting the return pass.
For the first time in the match, Arsenal looked fluent and they could have wrapped up the points when the ball fell to Van Persie in the box but the Dutchman cracked his shot against the post.
Eboue also had a chance to score Arsenal's third but Davies made a terrific block to deny him.
Late equaliser 'frustrating' - Wenger
Then at the other end, Sagna somehow nicked the ball off Ashley Young's toes as he looked set to convert Sidwell's cross.
Just when it seemed Villa would get nothing at all from the game, they won a penalty when Gallas upended Agbonlahor and Barry smashed in the resultant spot-kick.
Milner was then denied by Silvestre, who got in another vital block, as Villa pressed for a leveller.
And with time running out, the hosts earned their draw when Stiliyan Petrov's cross fell to defender Knight and he rifled the ball in.
Aston Villa: Friedel, Reo-Coker, Davies, Knight, Luke Young, Milner, Sidwell, Petrov, Barry, Ashley Young, Agbonlahor.
Subs Not Used: Guzan, Harewood, Delfouneso, Salifou, Shorey, Gardner, Osbourne.
Booked: Agbonlahor, Reo-Coker, Petrov, Barry.
Goals: Barry 65 pen, Knight 90.
Arsenal: Almunia, Sagna, Eboue, Toure, Silvestre, Song Billong (Ramsey 43), Denilson, Nasri (Clichy 82), Diaby, Gallas, Van Persie.
Subs Not Used: Fabianski, Vela, Wilshere, Bendtner.
Booked: Song Billong, Toure, Diaby, Van Persie.
Goals: Denilson 40, Diaby 49.
Ref: Lee Mason (Lancashire).
BBC Sport Player Rater man of the match: Aston Villa's Ashley Young 7.49 (on 90 minutes).
Please note that you can still give the players marks out of 10 on BBC Sport's Player Rater after the match has finished.
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