If you have a Subversion repository setup with multiple top-level projects and their typical branches/tags/trunk setup and want to migrate these to individual Mercurial (Hg) repositories, you can do this with the convert extension.
First you need to enable convert in your .hgrc by adding a section like the following:
Next, if needed, create a plain-text file, e.g. author-map.txt, containing SVN username to Hg author mappings, e.g. asmodai=Jeroen Ruigrok van der Werven <email@example.com>.
Next run Hg as follows:
% hg --authors author-map.txt --config convert.svn.branches=project/branches
--config convert.svn.tags=project/tags --config
This will start a SVN to Hg conversion, picking up only the changes and commit messages applicable for the various paths you gave for the branches, tags, and trunk, effectively splitting off this project from the main SVN tree into its own Hg repository.
Do note that for large SVN repositories this might not be the most efficient conversion way forward. In that case converting once from SVN to Hg and then split off Hg into many Hg repositories might be faster. Will adjust this post when I write that up.