FRONTEND_MVC/modules/sitemap/views/news.php

<?php

/* @var $this \yii\web\View */
/* @var $content string */

//use yii\helpers\Html;
Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
Yii::$app->response->headers->set('content-type', 'application/xml;charset=utf-8');
$this->beginPage();
echo $content;
$this->endPage();