Is there any nagative impact on CPU or IO when we apply MVC.Also,is decompression applies to MVC at all while fetching data.I assume not as values stored in table header.
I have never seen any report which says that MVC caused an increase in IO or CPU (assuming of course that your table does get smaller!). Generally, if you make the table smaller with MVC you will save CPU and IO.
If a column is compressed using MVC (and I think ALC) then the dbms will try and keep it compressed for as long as possible during query processing, i.e. storing the compressed version in spool files.