This seems to work for me -
declare @dt datetime = '2012-11-22 06:50:10.000'
select convert(varchar(20),@dt,120) -- result is 2012-11-22 06:50:10
And here is a SQL Fiddle showing it: http://www.sqlfiddle.com/#!3/b9bb1/7
This -
convert(nvarchar(20), '2012-11-22 06:50:10.000', 20)
does not work because the date is just a varchar