tv.setTypeface(null,Typeface.NORMAL);
This will restore the style to normal without changing color or size.
But you cannot mix bold / italic / underlined text this way. If you specify BOLD, all text will be in bold. If you want to mix text style, I suggest using HTML to style the text, and then use the following code.
tv.setText(Html.fromHtml(yourStringAsHtml));
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