when mouseover on username display popup views
Hope this will help you

In Drupal 7, for your requirement you can use qTip (Stylish jQuery Tooltips) module with Views.


After installing and configuring qTip, go through the qtip examples(refer qtip README.txt).
For this example I have created a view with some user fields, these fields will popup on hovering user picture
Create a new view and add fields which you want to show on the popup.

For showing user information in popup, you need to give the proper structure(read qtip README.txt). 
I have added the HTML structure for profile_image view field


On mouse hover of the user picture, user information will popup. And make CSS changes as per your requirement.
You can also refer BeautyTips, for example you can refer this link


Drupal 7 : Ajax load

Steps


To show user details on hover you need to add attributes to the username, it can be done using preprocess function template_preprocess_username in your active theme tempalte file.
 
function themeName_preprocess_username(&amp;$variables) {
  $variables['attributes_array'] = array('class' => array("user-info"), 'data-uid' => $variables['account']->uid);//adding more attributes for your requirement
}

Create a view with the fields you want to show on popup.
Create a menu to get user details like get/user_details/%, where % is the user-id. This menu will returns user details.

function test_menu() {
...
$items['get/user_details/%'] = array(
'title' => t(''),
'type' => MENU_CALLBACK,
'page callback' => 'get_user_details_ajax_callback',
'page arguments' => array(2),
'access arguments' => array('access content'),
);
...
return $items;
}
//Implement the ajax callback to get user details
function get_user_details_ajax_callback ($uid) {
$html = '';
$html = views_embed_view('VIEWNAME','display_id', $uid);
return $html;
}

For showing the user details on hover, you need to make a ajax call using 
$.ajax
Once the user hover the username get the data-uid attribute and make a ajax call to get user info.
Once you get the user details append it and show as popup.

Drupal.behaviors.getUserDetails = {
    attach: function (context, settings) {
      var replace_wrapper = 'user-'+$(this).attr('data-uid')+'details';
      //add html to show user details
      $( "a.user-info" ).each(function( index ) {
        if(!$(this).closest('div').hasClass('user-details-wrapper')){

      $(this).append("&lt;div class='user-details-wrapper'&gt;&lt;div class'"+replace_wrapper+"'&gt;&lt;/div&gt;&lt;/div&gt;");
    }
  });
  $('a.user-info').mouseenter(function(){
    if($(replace_wrapper).is(':empty')){//if empty then make ajax call and
      var url = Drupal.settings.basePath + 'get/user_info/'+ $(this).attr('data-uid');
      $.ajax({
        url: url,
        type: 'GET',
        success: function (data) {
          //create html and append to replace_wrapper
          var user_detail_html = '';
          //code to get user html here
          $(replace_wrapper).each(function( index ) {
            $(this).html(user_detail_html);
          });
        }
      });
    }else{
      //show popup here
      $('user-details-wrapper').show();
    }
  }).mouseleave(function() {
    //hide popup here
    $('user-details-wrapper').hide();
  });
 }
}



Make changes as per your requirement

Note : views_embed_view