"This is a massive chance for me to go and play for a good club like Reading and prove my worth and then come back here and obviously try and do the same here," he added.
"Brendan Rodgers, who is the manager at Reading, had tried to get me when he was at Watford, and I know a lot about him. I've spoken to him and he's a great manager.
"I also know a couple of the Irish lads down there as well and they've had nothing but good things to say."
New Zealand international Killen, whom Mowbray signed for Hibernian from Oldham in January 2006, spent a frustrating loan spell at Norwich City in the second half of last season.
But he impressed the new Celtic manager in the pre-season match against Brisbane Roar, scoring twice in a 3-0 win.
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