admin ajax php success returns 0

Add die(); to the end of the function handle ajax.
Add return false; after jQuery.ajax

function tw_pages_results() {
    //stuff
    die(); }

jQuery(document).ready(function() {
jQuery.ajax({
    type : "post",
    dataType : "html",
    url : "<?php echo site_url(); ?>/wp-admin/admin-ajax.php",
    data : {action : "tw_pages_results", 'page':0, 'itemsperpage':<?php echo $item_per_page; ?>, nonce: "<?php echo $tw_pages_nonce; ?>"},
    success: function(response) {
        jQuery("#results").html(response);
        jQuery("#1-page").addClass('active');
    }
});
return false;
});