According to the documentation of PropertyValueException, if you set a null value in a property declared not-null="true" and then try to persist/update it, then you will get this exception.
See http://docs.jboss.org/hibernate/orm/3.5/javadoc/org/hibernate/PropertyValueException.html
You should remove the not-null="true" attribute from the config (because the default value of not-null is false) and then the database should insert the default value that you specified in the configuration for you (in this case '1').
[UPDATE]
I would have preferred to put this as a comment in fujy's answer but I am not allowed to comment on other answers yet. Does it work as expected if you remove the 'not-null' attribute (As I am showing below)? I believe that is what is causing the exception that you are getting.
<property name="isActive" type="java.lang.Short">
<meta attribute="default-value">1</meta>
<column name="IsActive" />
</property>