Use the image element and reference your SVG file. For fun, save the following as recursion.svg:
<svg width="100%" height="100%" viewBox="-100 -100 200 200" version="1.1"
xmlns="http://www.w3.org/2000/svg">
<circle cx="-50" cy="-50" r="30" style="fill:red" />
<image x="10" y="20" width="80" height="80" href="recursion.svg" />
</svg>