var newline = System.Environment.NewLine;
    var tab = "\t";
    var clipboard_string = "";

    foreach (DataGridViewRow row in dataGridView1.Rows)
    {
         for (int i=0; i < row.Cells.Count; i++)
         {
              if(i == (row.Cells.Count - 1))
                   clipboard_string += row.Cells[i].Value + newline;
              else
                   clipboard_string += row.Cells[i].Value + tab;
         }
    }

    Clipboard.SetText(clipboard_string);