如何包含点击的表单提交按钮的值？

I must submit a form and I need to include the value of the submit button that was clicked. How can I do that? This is my code:

<form id="form_update" >
    <input type="text" class="form-control" name="title" value="" >
    <input type="hidden" name="prova" value="prova" />
    <button type="submit" name="salva" value="delete" class="btn btn-success">delete</button>
    <button type="submit" name="salva" value="modify" class="btn btn-success">modify</button>
    <button type="submit" name="salva" value="save" class="btn btn-success">save</button>
</form>

$(document).on('submit', '#form_update', function() { 
    return callAjax($(this).serialize()); 
}); 

function callAjax(data) {
    $.ajax({
        type: 'POST',
        url: 'call/function.php',
        data: data,
        success:  function(data) { 
            // code
        },
        error: function(data) { 
            alert('error'); 
        }
    });
    return false;
};  

Thanks...

With Arun solution I get undefined val value...

With Daan solution the problem was the same...

But with your response I have resolved with a mix :)

$(document).on('click', 'button', function(e) {
  e.preventDefault(); // Prevent from submitting form. 
  var buttonValue = $(this).val();
  return callAjax($('#form_update').serialize()+'&salva='+buttonValue ); 
}); 

Thanks a lot ;)

I would change the event type to a click on the button and the access their values with $(this).val()

$(document).on('click', 'button', function(e) {
    e.preventDefault(); // Prevent from submitting form. 
    var buttonValue = $(this).val();
    return callAjax($('#form_update').serialize()); 
}); 

Use the below function.

$(document).on('submit', '#form_update', function() { 
    var val = $("input[type=submit][clicked=true]").val();
    return callAjax($(this).serialize()+'&clickedVal='+val ); 
}); 

It will pass the clicked button value with the ajax request.