You are modifying collection from which you are deleting, try following in button1_Click:
List<DataGridViewRow> rowsToRemove = new List<DataGridViewRow>();
foreach (DataGridViewRow row in Dgv.Rows)
{
if (Convert.ToBoolean(row.Cells[0].Value))
{
rowsToRemove.Add(row);
}
}
rowsToRemove.ForEach(row => Dgv.Rows.Remove(row));