Quite simply, should a sentence read "a XML report" or "an XML report"?
It's "an XML report", because which form of the indefinite article to use is decided by how the next word is pronounced. In this case, it's pronounced:
An ex-emm-ell report
'XML' begins with a vowel sound, 'e', so an is used rather than a.
"An XML report" - because it is eXeMel - the sound starts with a vowel.