<p>How to move a sub directory from one directory to another, given that both are inside the same SVN repository? I am using tortoise SVN.</p>

Select the folder/files that you want to move, then RIGHT-CLICK them and drag them to where you want to move them.  A menu will come up with an option "
SVN Move versioned files here
"
As clarified by
phenry
, the destination folder must be recognized as an SVN folder.  The folder should already be committed to the repository or you can choose Add to set it to be added.
If the destination folder isn't part of an SVN repository, use the SVN Export command after your right-click and drag to copy the files to any destination folder.