Thanks for your reply Dhaval.  I've tried that way, but didn't work. I need to do DML operation before making callout in order to use the dml value to make callout .  I'm new to salesforce and pls let me know how to fix this.

Thank you

Thanks for the reply Maf_007.  I am new to salesforce and could you please show me how to write the test method which skips the callout?  

Thank you

global class LeadCallOutBatchable implements Database.Batchable<sObject>, Database.AllowsCallouts{
global final String Query;
   global final String Entity;
 
   global LeadCallOutBatchable() {
      Query='SELECT Id, FirstName, LastName, UserName__c, Email, Title, fs_skills__c, fs_overview__c, fs_facebook__c, fs_twitter__c, fs_linkedin__c ' +   'FROM Lead WHERE UserName__c != null';  
        Entity= 'Lead';
   }
 
   global Database.QueryLocator start(Database.BatchableContext BC) {
      return Database.getQueryLocator(query);
   }
    
   global void execute(Database.BatchableContext BC, List<Lead> scope) {
        
        List<Lead> leadsToUpdate = new List<Lead>();
        String PersonURL ;
        Integer count = 0;
        System.debug('scope: ' + scope);
        
        for(Lead leadRec : scope ) {   
            String jResponse = '';
            JPersonList deserialPersonList = new JPersonList();
            count++;
            System.debug('Count: ' + count);           
            PersonURL =  URL
            
            try {
                HttpRequest req = new HttpRequest();
                req.setMethod('GET');
                String username = JUSERNAME;
                String password = JPW;
                Blob headerValue = Blob.valueOf(username + ':' + password);
                String authorizationHeader = 'Basic ' + EncodingUtil.base64Encode(headerValue);
                req.setHeader('Authorization', authorizationHeader);
                req.setHeader('Accept', 'application/json');
                req.setEndPoint(PersonURL);
                
                Http http = new Http();
                system.debug('req: ' + req);
             if (Test.isRunningTest()) { //ADD THIS BLOCK PLEASE
                HTTPResponse res = http.send(req);
             }
         
               jResponse = res.getBody();
                
 
                jResponse = jResponse.Replace('throw \'allowIllegalResourceCall is false.\';', '');    
                //System.debug('Request Body: ' + jResponse);
                
                JSONParser parser = JSON.createParser(jResponse);
 
  // deserialize
                JObject personRecord = (JObject) System.JSON.deserialize(jResponse, JObject.class);
 
//some codes to compare
                
                       leadsToUpdate.add(leadRec);
                       System.debug('leadsToUpdate: ' + leadsToUpdate);
            }
            }
            catch(Exception ex) {
                System.debug('Exception: ' + ex);   
            }
        }
        System.debug('leadsToUpdate: ' + leadsToUpdate);
        update leadsToUpdate;
    }
        
   global void finish(Database.BatchableContext BC){ 
   }
}
 
 
test classes:
 
@isTest
public class LeadCalloutBatchableTest{
    public static testmethod void TestLeadCalloutBatchable() {
        
        List<ID> acRecs=new List<ID>();
        Lead testLead = new Lead( IncludeInSFToSync__c = true, FirstName = 'STEVE' , LastName = 'JOHN', UserName__c = 'stevej', Email = 'r@gmail.com', Title = 'testtest', fs_skills__c = 'developement' , fs_overview__c='good', fs_facebook__c='facebook', fs_twitter__c='twitter', fs_linkedin__c = 'linkedin');
        insert testLead;
        acRecs.add(testLead.id);
       
        string JSONstr = 
        SingleRequestMock fakeResponse = new SingleRequestMock(200,'Complete', JSONstr ,null);
 
        Test.startTest();
        Test.setMock(HttpCalloutMock.class, fakeResponse);
        Database.executeBatch(new LeadCallOutBatchable(), 200);
       
        Test.stopTest();
   
    }
}
 
 
@isTest
global class SingleRequestMock implements HttpCalloutMock {
    protected Integer code;
    protected String status;
    protected String bodyAsString;
    protected Blob bodyAsBlob;
    protected Map<String, String> responseHeaders;
  
   
    public SingleRequestMock(Integer code, String status, String body, Map<String, String> responseHeaders) {
        this.code = code;
        this.status = status;
        this.bodyAsString = body;
        this.bodyAsBlob = null;
        this.responseHeaders = responseHeaders;
 
    }
         
   public SingleRequestMock(Integer code, String status, Blob body, Map<String, String> responseHeaders) {
        this.code = code;
        this.status = status;
        this.bodyAsBlob = body;
        this.bodyAsString = null;
        this.responseHeaders = responseHeaders;
    }
    
    public HTTPResponse respond(HTTPRequest req) {
        HttpResponse resp = new HttpResponse();
        resp.setStatusCode(code);
        resp.setStatus(status);
        if (bodyAsBlob != null) {
            resp.setBodyAsBlob(bodyAsBlob);
        } else {
            resp.setBody(bodyAsString);
        }
        
        if (responseHeaders != null) {
            for (String key : responseHeaders.keySet()) {
                resp.setHeader(key, responseHeaders.get(key));
            }
        }
        return resp;
    }  
}

Could you please add the block I added in your code and see what happens??