Hmm.. Lots of trials trials and.. at Last I found the solution....

<apex:page controller="SelectList3">
<apex:form >
<apex:pageBlock >
  <apex:pageBlockSection title="Contact List">
    <apex:selectList value="{!slv}" size="1" label="Contaclist">
        <apex:actionSupport event="onchange" reRender="table" />
        <apex:selectOptions value="{!value}">                     
        </apex:selectOptions>               
    </apex:selectList>
   </apex:pageBlockSection>
 </apex:pageBlock>
</apex:form>
<apex:pageBlock title=" Value of contacts">
<apex:pageBlockSection >
<apex:outputPanel id="out">
   <apex:pageblockTable value="{!tabledata}" var="c" cellpadding="2px" id="table">
       <apex:column value="{!c.name}" headerValue="Account Name"/>
       
           
   </apex:pageblockTable>
   
</apex:outputPanel>
</apex:pageBlockSection>

</apex:pageBlock> </apex:page>

------------------------ Controller------------------------------------------

public with sharing class SelectList3 

{

public list<Account> getTabledata()
 {           
      list<Account> con=[select name from Account where id=:slv];        
      return con;
 }


   public list<selectoption> getValue()
 {
    list<SelectOption> ls= new list<SelectOption>();
    for(Account acc:[select id, name from Account])
    {
        ls.add(new SelectOption(acc.id,acc.name));
    }
    return ls;
}

public string slv{get;set;}

}

After a lot of trials, found the solution....

<apex:page controller="SelectList3">
<apex:form >
<apex:pageBlock >
  <apex:pageBlockSection title="Contact List">
    <apex:selectList value="{!slv}" size="1" label="Contaclist">
        <apex:actionSupport event="onchange" reRender="table" />
        <apex:selectOptions value="{!value}">                     
        </apex:selectOptions>               
    </apex:selectList>
   </apex:pageBlockSection>
 </apex:pageBlock>
</apex:form>
<apex:pageBlock title=" Value of contacts">
<apex:pageBlockSection >
<apex:outputPanel id="out">
   <apex:pageblockTable value="{!tabledata}" var="c" cellpadding="2px" id="table">
       <apex:column value="{!c.name}" headerValue="Account Name"/>
       
           
   </apex:pageblockTable>
   
</apex:outputPanel>
</apex:pageBlockSection>

</apex:pageBlock> </apex:page>

------------------------ Controller------------------------------------------

public with sharing class SelectList3 
{

public list<Account> getTabledata()
 {           
      list<Account> con=[select name from Account where id=:slv];        
      return con;
 }


   public list<selectoption> getValue()
 {
    list<SelectOption> ls= new list<SelectOption>();
    for(Account acc:[select id, name from Account])
    {
        ls.add(new SelectOption(acc.id,acc.name));
    }
    return ls;
}

public string slv{get;set;}

}