The Dane's move was confirmed yesterday.
Helenius, who has two senior international caps, came through the youth ranks at Aalborg and the 6ft 5in frontman was the second-highest scorer in the Danish league last season with 16 goals.
"I learnt a lot over the past few seasons with (Aalborg coach and former Villa defender) Kent Nielsen as my manager," he said.
"He was a player here at Villa and, while I have always been a good finisher, he taught me to be more aggressive and determined when I don't have the ball.
"He showed me how to work hard and this will help me in this league.
"I thought that maybe when I was 28 or something I might eventually be able to move to the Premier League having made my mark in Danish football, but now I have my chance and I think this is the right club for me with all the other young players in the side who want to do well and achieve something together.
"I want to show the fans that I'm hungry for this and that I'm very proud to be a Villa player."