How add CVV's custom validation before order review in Ubercart
You can create a custom validation function in your module and name it whatever you like for example 'custom_validation_function'. and in the same module add this validation function to the form's validation list by using form_alter.

function &lt;module_name&gt;_form_alter(&amp;$form, &amp;$form_state, $form_id) {

  if ($form_id == '&lt;ubercart_form_id&gt;') {
    $form['#validate'][] = 'custom_validation_function';
  }
}