<servet>这个单词写错了，是这对标签<servlet></servlet>

<?xml version="1.0" encoding="UTF-8"?>

<web-app version="2.5" 

xmlns="http://java.sun.com/xml/ns/javaee" 

xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 

xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 

http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">

  <display-name></display-name>

  <welcome-file-list>

    <welcome-file>index.jsp</welcome-file>

  </welcome-file-list>

  <servet>

  <servlet-name>HelloServlet</servlet-name>

  <servlet-class>servlet.HelloServlet</servlet-class>

  </servet>

  <servlet-mapping>

  <servlet-name>HelloServlet</servlet-name>

  <url-pattern>/servlet/HelloServet</url-pattern>

  </servlet-mapping>

</web-app>

贴一下你的web.xml代码，或者检查一下servlet的jar包是否导入