This answer gives a code sample for drawing text with different colors into a picture box. You could easily modify it to draw the text with different fonts as well. If you need the scrollability of the RichTextBox, you could place the picture box on a panel.
Update: since you need to use a RichTextBox, this link shows you how to transform your original XML into RTF that you can load into your RichTextBox. In order to do this, you have to create an XSLT document that describes how to transform the XML into RTF. The link I included gives a sample XSLT document; you would have to modify this XSLT based upon how the different fonts are specified in your original XML document (if you post a sample of your original XML, we could probably help you modify the XSLT accordingly).