Preview option for entity registration form
I have one another option that i have used: 

function your_Module_name_form_alter(&amp;$form, &amp;$form_state, $form_id) {
   if ($form_id == 'your_form_id') { 
       $form['submit']['#attributes'] =&gt; array('class' =&gt; array('upersubmit'),'onclick' =&gt; 'if(!confirm_vil(this.id)){return false;}');
   }


}

/* Js Function in your js file */

function confirm_vil(val){
    /* Now here you can add popup code to view all data before submit */
}


i hope this is help you..