What is the difference between XML Schema (XSD) and XMI?
It's a bit like the difference between a dictionary and a gardening catalogue. In other words, they have very little in common.
XML Schema is a way of describing the structure and constraints on a class of XML documents.
XMI is an XML representation of UML models.
If you don't understand the difference, then you haven't understood one of these specifications, or perhaps you haven't understood either of them.