Not that it is impossible; it makes no
certain sense. In other words, you Question does not define any sense in which such "conversion" (what an improper term!) should be done.
XSD (XML Schema Definition) is a way to describe the meta-data schema. For example, it can be used to describe HTML standard (strictly speaking, only XHTML). HTML represent data which should be written in a way compliant with (X)HTML standard. In other words, one single XSD is supposed to describe a whole big set of different XMLs (theoretically, infinite but countable set). We came to conceptual mismatch.
However, any meta-data is a sort of data, and XSD is not an exclusion, it is itself is written in XML, and any XML can be rendered as some mark-up, which is possible to be implemented as HTML and rendered the way one can read and understand XSD as a schema. Warning: this is just an example of naive "conversion" which is no way the only solution serving any particular purpose (except showing the XSD in a human-readable way); the "real" purpose is unknown to me; and your Question suggests that you don't clearly understand it yourself.
Replace all '<', '>' and '&' characters by HTML character entities <, > and &, respectively and sandwich the contents of the XSD file in <pre></pre> tags. That's it.
—SA
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