跨域JavaScript Ajax

I'm dealing with a problem the whole week already and can't find the answer anywhere.

This is the problem:

XMLHttpRequest cannot load http://www.websiteA.com/process.php. Origin http://clientwebsi.te is not allowed by Access-Control-Allow-Origin.

I have a Javascript file that is located at the server from website A. Clients can load the JS file on there website.

On the server A is also a process.php located that puts information in the database on server A.

I'm using now this code:

var dataText = 'page=' + top.location.host;
$.ajax({
    type: "POST",                 
    url: "process.php",         
    data: dataText,              
    error: function(request,error){
            alert(error);
        },
    success: function(request) {
        alert(request.length);
    }
});

This code works perfectly on localhost but not when i'm using the server A and client server (cross domain)

This is the online code:

$.ajax({
    type: "POST",                 
    url: "http://www.serverA.com/process.php",         
    dataType: "json",
    data: dataText,              
    error: function(request,error){
            alert(error);
        },
    success: function(request) {
        alert(request.length);
    }
});

$.ajax({
    type: "POST",                 
    url: "http://www.serverA.com/process.php",         
    dataType: "jsonp",
    data: data,  
    crossDomain: true,            
    error: function(request,error){
            alert(error);
        },
    success: function(request) {
        alert(request.length);
    }

The only way is to make jsonp GET requests. It's quite easy but you can't use other request types.

$.ajax({
    dataType: 'jsonp',
    url: 'http://domain.de/jsonp.php',
    success: function(data, textStatus, jqXHR),
    error: function(jqXHR, textStatus, errorThrown)
});