When you use servlet api beyond 3.0 version, you can use java based configuration of your webapp instead of web.xml file.
Maven by default report an error when you try to compile project that does not contain web.xml file, so you have to additionally configure maven-war-plugin in pom.xml.

<plugin>
    <groupid>org.apache.maven.plugins</groupid>
    <artifactid>maven-war-plugin</artifactid>
    <version>2.2</version>
    <configuration>
        <failonmissingwebxml>false</failonmissingwebxml>
    </configuration>
</plugin>