Try This

Response.Clear()
            Response.Charset = ""
            Response.AddHeader("content-disposition", "attachment;filename=report.xls")
            Response.ContentType = "application/vnd.xls"
            Response.Cache.SetCacheability(HttpCacheability.NoCache)
            Dim stringWrite As New System.IO.StringWriter()
            Dim htmlWrite As System.Web.UI.HtmlTextWriter = New HtmlTextWriter(stringWrite)
            Excel_Word_details_view.RenderControl(htmlWrite)
            Response.Write(stringWrite.ToString())
            Response.End()
          Catch ex As Exception

Ya i do this but it give me error that it must be placed inside the form tag but i have one form tag on my master page so m not able to apply form tag here

Server Error in '/' Application.

Control 'MainContent_DetailsView1' of type 'DetailsView' must be placed inside a form tag with runat=server.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.  Exception Details: System.Web.HttpException: Control 'MainContent_DetailsView1' of type 'DetailsView' must be placed inside a form tag with runat=server. Source Error: 

Line 38:             DetailsView1.AllowPaging = false;
Line 39:             DetailsView1.DataBind();
Line 40: DetailsView1.RenderControl(hw); Line 41:             Response.Output.Write(sw.ToString());
Line 42:             Response.Flush();

Add This

public override void VerifyRenderingInServerForm(Control control)
{
/* Verifies that the control is rendered */
}

I have done this now different error occur something like that below

RegisterForEventValidation can only be called during Render();

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.  Exception Details: System.InvalidOperationException: RegisterForEventValidation can only be called during Render(); Source Error: 

Line 38:             DetailsView1.AllowPaging = false;
Line 39:             DetailsView1.DataBind();
Line 40: DetailsView1.RenderControl(hw); Line 41:             Response.Output.Write(sw.ToString());
Line 42:             Response.Flush();

This error occurs whenever we are trying to render control to response. To solve this problem I have added EnableEventValidation="false" to @Page directive of aspx page that should be just like this 

<%@ Page Language="C#" AutoEventWireup="true" CodeFile="ExportGridviewtoExcel.aspx.cs" Inherits="ExportGridviewtoExcel" EnableEventValidation="false"%>

By Setting EnableEventValidation="false" property to @Page directive of aspx page the problem has solved automatically.