Liquibase supports XML as a format for storing your changelog files. There is more information about the XML format in the FAQ.
XSD Support
XSD schema definitions are available for each Liquibase version. Since there are no changelog format changes in patch versions, there are only xsd files that correspond to major.minor versions.
If you use a Liquibase extension that includes additional change tags, check the extension documentation to find out if they provide a XSD.
If they do not, you can use the xsd at dbchangelog-ext.xsd which allows any nested tag and attribute.