XML Tutorial/XML Schema/nonNegativeInteger

The nonNegativeInteger datatype is derived from the integer type.

The value space is the infinite set of integers greater than or equal to zero. 
The lexical space is a finite-length sequence of decimal digits with an optional leading - or +.
<!-- schema -->
<xsd:element name="bank_balance"
  type="xsd:nonNegativeInteger"/>
<!-- instance document -->
<bank_balance>32</bank_balance>

Use nonNegativeInteger

File: Schema.xml
<?xml version="1.0"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
  targetNamespace="http://www.wbex.ru" xmlns="http://www.wbex.ru"
  elementFormDefault="qualified">
  <xs:element name="population" type="xs:nonNegativeInteger" />
</xs:schema>

File: Data.xml
<?xml version="1.0"?>
<population xmlns="http://www.wbex.ru">342</population>