Thanks for the replies.

One more doubt about the downtime

"Downtime is the time to take the backup and restore into the new server and changing the compatibility levels and fixing the orphan users issues"

which option is better for databases recovery in side by side migration. If both sql 2008 r2 & 2012 will be in the same network and we need less downtime

1)Back up the databases from the original server and then restore them to the SQL Server 2012 instance. 2)Manually detach your database from the old instance and reattach it to the new instance

3)Use log shipping, or database mirroring.

4)Using the Copy Database Wizard to migrate databases to the new server.