In web.xml file add servlet-name, servlet-class and servlet mapping tags.
<servlet>
<servlet-name>HelloServlet</servlet-name>
<servlet-class>com.test.servlet.HelloServlet</servlet-class> /* package name of your HelloServlet */
</servlet>
<servlet-mapping>
<servlet-name>HelloServlet</servlet-name>
<url-pattern>/HelloServlet</url-pattern>
</servlet-mapping>
deploy both the projects independently and in your browser try the
http://localhost:8080/YourWARProjectName/HelloServlet