SVGSVG is a graphical format which is scalable. It means that when you zoom an image, you don't loose quality. So this format gives good printing result.
SVG support in Web Browser is not very good, (see Support for SVG in web browsers). If you see the following image, it means that your browser does support SVG.
You can enable it by using
-tsvg flag with the command line.
You can also use
format="svg" in the ant task definition.
<target name="main"> <plantuml dir="./src" format="svg" /> </target>