If you're on Linux, or have cygwin available on Windows, you can run the input XML through a simple sed script that will replace <Active>True</Active> with <Active>true</Active>, like so:
cat <your XML file> | sed 'sX<Active>True</Active>X<Active>true</Active>X' | xmllint --schema -
If you're not, you can still use a non-validating xslt pocessor (xalan, saxon etc.) to run a simple xslt transformation on the input, and only then pipe it to xmllint.
What the xsl should contain something like below, for the example you listed above (the xslt processor should be 2.0 capable):
<?xml version="1.0"?>
<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
<xsl:output method="xml" indent="yes"/>
<xsl:template match="/">
<xsl:for-each select="XML">
<xsl:for-each select="Active">
<xsl:value-of select=" replace(current(), 'True','true')"/>
</xsl:for-each>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>