I know this is a fairly old thread but I have to wonder now why someone hasn't yet taken the svg rendering library/engine that the open-source linux program 'Gwenview' (and as far as I know many other linux programs) uses and ported it to make a free plugin for XnView that can display SVG files.
If I possessed the necessary programming skills I would attempt to do it, but my programming skills are limited to simple things written in Visual Basic and a very very small amount of old Turbo Delphi.
As far as I know another option might be to make a plugin based on the webkit rendering engine as well, I know there's an ongoing project on the main webkit site to incorporate svg rendering capabilities into webkit. And with the increasing popularity of svg graphics it would really be beneficial to many XnView users to have access to a free svg plugin (even if it didn't handle all of the complexities of the svg format).
I have managed to get the support for svg going on the portable version of xnview as well.
I downloaded the plugin from http://www.cadsofttools.com/download/cs_svg.zip. After extraction I copied CS_SVG.dll into the plugins folder of xnview which was at XnViewPortable\App\XnView\Plugins and voila I can now view the svg files using xnview.