Hi Arjun,
You are right with regards to the mapping in the web.xml.
<servlet>
<servlet-name>HelloWorld</servlet-name> HelloWorld is the java file name
<servlet-class>Users.Ravi.Desktop.CurrentProject.Servlets.HelloWorld</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>HelloWorld</servlet-name>
<url-pattern>/hello</url-pattern>
</servlet-mapping>
but the only problem is url which you are using to call the servlet. it should be in the following way
http://localhost:8080/HelloWorld/hello in the above URL you have different parts which are necessary.
http://<servername>:<port>/<context name>/<url-pattern or the name of the jsp>
The context name is the name of the folder of your application in the webapps directory of the tomcat.