Here is one way you could do it in XSLT 1.0.
First, find the first non-zero element, by removing all the zero elements currently in the value
<xsl:variable name="first" select="substring(translate(., '0', ''), 1, 1)" />
Then, you can find the substring-before this first character, and then use substring-after to get the non-zero part after this
<xsl:value-of select="substring-after(., substring-before(., $first))" />
Or, to combine the two statements into one
<xsl:value-of select="substring-after(., substring-before(., substring(translate(., '0', ''), 1, 1)))" />
So, given the following input
<a>00012095Kb</a>
Then using the following XSLT
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
<xsl:template match="/a">
<xsl:value-of select="substring-after(., substring-before(., substring(translate(., '0', ''), 1, 1)))" />
</xsl:template>
</xsl:stylesheet>
The following will be output
12095Kb