Schema versions can be found here.
Starting from version 6.1, MRV files begin with a reference to the schema to which the file is conformed:
<?xml version="1.0" encoding="UTF-8"?> <cml xmlns="http://www.chemaxon.com" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.chemaxon.com/marvin/help/formats/schema/mrvSchema_6_1_0.xsd" version="ChemAxon file format v6.1, generated by v6.1.0">
In previous versions, MRV format didn't have a reference to a schema:
<cml version="ChemAxon file format v6.0, generated by v6.0.0">
For these files, the recommended schema to use for validation is that of version 6.0.0.