How to describe the difference between two XML formats?

I would like to write a DocBook document describing the difference
between two XML formats.  One of the formats is a replacement for the
other format, and the target audience is the people currently
generating the old format.

My thought was to display an example in the old format, side by side
with an equivalent example in the new format, and then explain the
difference between them in prose.

My questions are: are there any tools I can use to generate the side
by side example?  Some sort of XML diff program, perhaps?  

If not, is it possible to have two <mediaobject>s placed side by side,
with a listing in each?  Or do I have to manually put the examples
side by side in a text file to be included?

Is there a way to intermingle lines in the the examples with
annotating text? (I did that when generating code examples using the
Qt qdoc tool, and the result was highly effective)

Thanx!


- Steinar