is there any app for running .mkv , .divx, .flv , .avi etc etc formats???
You do realise the MKV, AVI and DivX are just containers for actual audio and video files? They aren't video formats in their own right. For instance, a DivX video would actually contain a H.264/MPEG-4 AVC video file which WP 8 can play. What you need to do then is use a free (or paid) video converter in order to play the aforementioned files.
For flv or Flash videos WP 8 doesn't support this format natively but there is an app in the store that allows us to watch and download Flash content online.
People always say "just convert it.". Some of us actually utilize the features provided by mkv like subtitle and chapter support. Why do video software makers always ignore this format when it is the only one that properly supports the features? Very annoying.