This Way:
HTML:
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
    <title></title>
    <style type="text/css">
        .modalPopup
        {
            background-color: #FFFFFF;
            border-width: 3px;
            border-style: solid;
            border-color: black;
            padding-top: 10px;
            padding-left: 10px;
            width: 300px;
            height: 50x;
        }
    </style>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <cc1:ToolkitScriptManager runat="server">
        </cc1:ToolkitScriptManager>
        <asp:DetailsView ID="dvDetails" runat="server" AutoGenerateRows="false" OnDataBound="dvDetails_DataBound">
            <Fields>
                <asp:TemplateField HeaderText="Name">
                    <ItemTemplate>
                        <asp:Label ID="lblName" runat="server" Text='<%# Eval("Name") %>'></asp:Label>
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:TemplateField HeaderText="Question">
                    <ItemTemplate>
                        <asp:Label ID="lblQuestion" runat="server" Text='<%# Eval("Question") %>'></asp:Label>
                        <asp:Panel ID="pnlDemo" CssClass="modalPopup" runat="server">
                            <asp:Label ID="lblLongAddress" Text='<%# Eval("Question") %>' runat="server"></asp:Label>
                        </asp:Panel>
                        <cc1:HoverMenuExtender ID="hmeDemo" runat="server" PopupControlID="pnlDemo" PopupPosition="Bottom"
                            TargetControlID="lblQuestion">
                        </cc1:HoverMenuExtender>
                    </ItemTemplate>
                </asp:TemplateField>
            </Fields>
        </asp:DetailsView>
    </div>
    </form>
</body>
</html>
C#:
protected void Page_Load(object sender, EventArgs e)
        {
            if (!this.IsPostBack)
            {
                DataTable dt1 = new DataTable();
                dt1.Columns.AddRange(new DataColumn[2] { new DataColumn("Name", typeof(string)),
                                                         new DataColumn("Question", typeof(string)) 
                                                        });
                dt1.Rows.Add("Babu", "I want to display compnay details like company id, name, description, etc in DetailsView. The descrption is took long. So, I want to display first 100 characters from description initially and when user mouse over and want to display complete description in a popup. How to proceed to this kind of functionality ??");
                this.dvDetails.DataSource = dt1;
                this.dvDetails.DataBind();
            }
        }
        protected void dvDetails_DataBound(object sender, EventArgs e)
        {
            Label question = dvDetails.FindControl("lblQuestion") as Label;
            string newQuestion = question.Text.Substring(0, 100);
            question.Text = newQuestion;
        }
Image:

Here when i MouseOver the second cell of row 2 it shows this complete Question.