How is the internal implementation of LinkedHashMap different from HashMap implementation?

HashMap does not maintain insertion order, hence it does not maintain any doubly linked list.

Most salient feature of LinkedHashMap is that it maintains insertion order of key-value pairs. LinkedHashMap uses doubly Linked List for doing so.

Entry of LinkedHashMap looks like this-

  static class Entry<K, V> {
     K key;
     V value;
     Entry<K,V> next;
     Entry<K,V> before, after;        //For maintaining insertion order    
     public Entry(K key, V value, Entry<K,V> next){
         this.key = key;
         this.value = value;
         this.next = next;
     }
  }

By using before and after – we keep track of newly added entry in LinkedHashMap, which helps us in maintaining insertion order.

Before refers to previous entry and
after refers to next entry in LinkedHashMap.

For diagrams and step by step explanation please refer http://www.javamadesoeasy.com/2015/02/linkedhashmap-custom-implementation.html

Thanks..!!