XML in Yii2, Option 1

public function actionTest() {
    \Yii::$app->response->format = \yii\web\Response::FORMAT_XML;
    return $this->renderPartial('test');
}

In line 2, the code \yii\web\Response can be moved to use, then the code will be as follows: 

<?php

//...

use yii\web\Response;

//...

public function actionTest() {
    \Yii::$app->response->format = Response::FORMAT_XML;
    return $this->renderPartial('test');
}

XML in Yii2, Option 2

With help behaviors: 

<?php

//...

public function behaviors() {
    return [
        [
            'class' =>  \yii\filters\ContentNegotiator::className(), //can be moved to use yii\filters\ContentNegotiator
            'only' => ['xml'], //action name
            'formats' => [
                'text/xml' => Response::FORMAT_XML,
            ],
        ],
    ];
}

public function actionTest() {
    return $this->renderPartial('test');
}

Here \yii\filters\ContentNegotiator can be moved to use. Also, consider an option when the behaviors method already exists: 

<?php

//...

namespace frontend\controllers;

//...

use yii\filters\ContentNegotiator;
use yii\web\Response;

class SiteMapController extends BaseController
{

    public function behaviors() {
        return [
            'access' => [
                'class' => AccessControl::className(),
                'only' => ['logout'],
                'rules' => [
                    [
                        'actions' => ['logout'],
                        'allow' => true,
                        'roles' => ['@'],
                    ],
                ],
            ],
            'verbs' => [
                'class' => VerbFilter::className(),
                'actions' => [
                    'logout' => ['post'],
                ],
            ],
            [
                'class' => ContentNegotiator::className(),
                'only' => ['test'],
                'formats' => [
                    'text/xml' => Response::FORMAT_XML,
                ],
            ],
        ];
    }

    //...

    public function actionTest() {
        return $this->renderPartial('test');
    }
}

XML in Yii2, Option 3

public function actionTest() {   
    Yii::$app->response->format = \yii\web\Response::FORMAT_RAW;
    Yii::$app->response->headers->add('Content-Type', 'text/xml');
    return $this->renderPartial('test');
}

\yii\web\Response can be moved to use.

It is worth noting that in all options, the view is displayed using renderPartial without the main layout.