Question

I need to hide the last row with some text (say click to insert new row) and when user clicks the text will dissaper and row will be added. and the text will apper move down to add another row.
it basically text block is overlapping on last row of Datagridview.

Answer 1

See the following link, it does exactly that
http://blogs.msdn.com/b/vinsibal/archive/2008/11/05/wpf-datagrid-new-item-template-sample.aspx

Update 2

Modified the ControlTemplate a bit and uploaded my sample project here:
http://www.mediafire.com/download.php?ea519xwbc53i91i

Update

Adding the necessary steps from the link

Xaml

<DataGrid x:Name="dataGrid"
          LoadingRow="dataGrid_LoadingRow"
          UnloadingRow="dataGrid_UnloadingRow"
          RowEditEnding="dataGrid_RowEditEnding"
          ...>
    <DataGrid.Resources>
        <ControlTemplate x:Key="NewRow_ControlTemplate" TargetType="{x:Type DataGridRow}">
            <Border x:Name="DGR_Border" Background="{TemplateBinding Background}" BorderBrush="{TemplateBinding BorderBrush}" BorderThickness="{TemplateBinding BorderThickness}" SnapsToDevicePixels="True">
                <SelectiveScrollingGrid>
                    <TextBlock Text="Click here to add a new item." Grid.Column="1"/>
                </SelectiveScrollingGrid>
            </Border>
        </ControlTemplate>
    </DataGrid.Resources>
    <DataGrid.RowStyle>
        <Style TargetType="DataGridRow">
            <EventSetter Event="MouseLeftButtonDown" Handler="Row_MouseLeftButtonDown" />
        </Style>
    </DataGrid.RowStyle>
    <!--...-->
</DataGrid>

Code behind

ControlTemplate _defaultRowControlTemplate = null;
ControlTemplate _newRowControlTemplate = null;
private void dataGrid_LoadingRow(object sender, DataGridRowEventArgs e)
{
    if (_defaultRowControlTemplate == null)
    {
        _defaultRowControlTemplate = e.Row.Template;
    }
    if (_newRowControlTemplate == null)
    {
        _newRowControlTemplate = dataGrid.FindResource("NewRow_ControlTemplate") as ControlTemplate;
    }
    if (e.Row.Item == CollectionView.NewItemPlaceholder)
    {
        e.Row.Template = _newRowControlTemplate;
        e.Row.UpdateLayout();
    }
}
private void dataGrid_UnloadingRow(object sender, DataGridRowEventArgs e)
{
    if (e.Row.Item == CollectionView.NewItemPlaceholder && e.Row.Template != _defaultRowControlTemplate)
    {
        e.Row.Template = _defaultRowControlTemplate;
        e.Row.UpdateLayout();
    }
}
private void Row_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
{
    DataGridRow row = sender as DataGridRow;
    if (row.Item == CollectionView.NewItemPlaceholder && row.Template == _newRowControlTemplate)
    {
        // for a new row update the template and open for edit
        row.Template = _defaultRowControlTemplate;
        row.UpdateLayout();
        dataGrid.CurrentItem = row.Item;
        DataGridCell cell = DataGridHelper.GetCell(dataGrid, dataGrid.Items.IndexOf(row.Item), 0);
        cell.Focus();
        dataGrid.BeginEdit();
    }
}
private void dataGrid_RowEditEnding(object sender, DataGridRowEditEndingEventArgs e)
{
    IEditableCollectionView iecv = CollectionViewSource.GetDefaultView((sender as DataGrid).ItemsSource) as IEditableCollectionView;
    if (iecv.IsAddingNew)
    {
        // need to wait till after the operation as the NewItemPlaceHolder is added after
        Dispatcher.Invoke(new DispatcherOperationCallback(ResetNewItemTemplate), DispatcherPriority.ApplicationIdle, dataGrid);
    }
}
private object ResetNewItemTemplate(object arg)
{
    DataGridRow row = DataGridHelper.GetRow(dataGrid, dataGrid.Items.Count - 1);
    if (row.Template != _newRowControlTemplate)
    {
        row.Template = _newRowControlTemplate;
        row.UpdateLayout();
    }
    return null;
}

DataGridHelper

public static class DataGridHelper
{
    public static DataGridCell GetCell(DataGrid dataGrid, int row, int column)
    {
        DataGridRow rowContainer = GetRow(dataGrid, row);
        if (rowContainer != null)
        {
            DataGridCellsPresenter presenter = GetVisualChild<DataGridCellsPresenter>(rowContainer);
            if (presenter == null)
            {
                dataGrid.ScrollIntoView(rowContainer, dataGrid.Columns[column]);
                presenter = GetVisualChild<DataGridCellsPresenter>(rowContainer);
            }
            // try to get the cell but it may possibly be virtualized
            DataGridCell cell = (DataGridCell)presenter.ItemContainerGenerator.ContainerFromIndex(column);
            if (cell == null)
            {
                // now try to bring into view and retreive the cell
                dataGrid.ScrollIntoView(rowContainer, dataGrid.Columns[column]);
                cell = (DataGridCell)presenter.ItemContainerGenerator.ContainerFromIndex(column);
            }
            return cell;
        }
        return null;
    }
    public static DataGridRow GetRow(DataGrid dataGrid, int index)
    {
        DataGridRow row = (DataGridRow)dataGrid.ItemContainerGenerator.ContainerFromIndex(index);
        if (row == null)
        {
            object hh = dataGrid.Items[index];
            // may be virtualized, bring into view and try again
            dataGrid.ScrollIntoView(hh);
            dataGrid.UpdateLayout();
            row = (DataGridRow)dataGrid.ItemContainerGenerator.ContainerFromIndex(index);
        }
        return row;
    }
    public static T GetVisualChild<T>(Visual parent) where T : Visual
    {
        T child = default(T);
        int numVisuals = VisualTreeHelper.GetChildrenCount(parent);
        for (int i = 0; i < numVisuals; i++)
        {
            Visual v = (Visual)VisualTreeHelper.GetChild(parent, i);
            child = v as T;
            if (child == null)
            {
                child = GetVisualChild<T>(v);
            }
            if (child != null)
            {
                break;
            }
        }
        return child;
    }
}