Interesting question, how a native Hindi speaker express “I love you” in Hindi?
What could be the variations? situations? And, what if the expression is directed toward a family member(s), friends and not the love relationships?

Let me show you, how to express it like we do. 🙂 I have separated the expressions for male and female for easy reference plus I have provided some important note.  If you want to say “much” or “very much” like “I love you very much”, just add a prefix “बहुत” (bahut) before “प्यार” (pyaar) in the sentences. You can take print out of this post, it can help you latter. 🙂

Male

1. Love/Romantic relationship – Male to Female (Hetrosexual) / Male to Male (Homosexual)

मैं तुमसे प्यार करता हूँ.
Main tum se pyaar karta hoon.

मुझे तुम से प्यार है
Mujhe tum se pyar hai.

मैं तुम को प्यार करता हूँ.
Main tum ko pyaar karta hoon.

मैं तुम्हे प्यार करता हूँ.
Main tumhe pyaar karta hoon.

Note : The above mentioned 4 expressions are those we use quite normally. These expressions are for two persons when they know each other and have close understanding.

मैं तुझे प्यार करता हूँ
Main tujhe pyaar karta hoon.

Note: “तुझे” (tujhe – to you) shows that the you/the person is closer to you in your/ her thoughts. Because “तुझे” (tujhe) comes in between “tum” (less formal) and “tu” (informal) for “you” in Hindi. “Tu” shows less respect but at the same time more closeness in relationship.

मैं आप से प्यार करता हूँ
Main aap se prem karta hoon.

Note: “आप” (Aap) shows that the you/the person is still at distance to you in your/ her thoughts or understanding. Because “आप” (Aap) is formal but sometime an indicator of more respect in the relationship.

मैं तुम से प्रेम करता हूँ
Main tum se prem karta hoon.

Note : “प्रेम” (Prem) could be heard as well. This word is less common in usage (because it is from pure Hindi) but could be found in Hindi literature and from the mouth of a Hindi Scholar. 🙂

2. Family or friends (Informal)

मैं तुम सब को प्यार करता हूँ.
Main tum ko pyaar karta hoon.
I love you all.

3. Family or friends (Formal)

मैं आप सब को प्यार करता हूँ
Main aap sab ko pyaar karta hoon.
I love you all.

Note: For both 2. & 3., to family and friends, please note the use of  “को” (ko), which is usually preferred than “से” (se) as latter is used commonly in love relationships.

Female

1. Love/Romantic relationship – Female to Male (Hetrosexual) / Female to Female (Homosexual)

मैं तुमसे प्यार करती हूँ.
Main tum se pyaar karti hoon.

मुझे तुम से प्यार है
Mujhe tum se pyar hai.

मैं तुम को प्यार करती हूँ.
Main tum ko pyaar karti hoon.

मैं तुम्हे प्यार करती हूँ.
Main tumhe pyaar karti hoon.

Note : The above mentioned 4 expressions are those we use quite normally. These expressions are for two persons when they know each other and have close understanding.

मैं तुझे प्यार करती हूँ
Main tujhe pyaar karti hoon.

Note : “तुझे” (tujhe – to you) shows that the you/the person is closer to you in your/ her thoughts. Because तुझे (tujhe) comes inbetween tum (less formal) and tu (informal). “Tu” shows less respect but at the same time more closeness in relationship.

मैं आप से प्यार करती हूँ
Main aap se prem karti hoon.

Note: “आप” (Aap) shows that the you/the person is still at distance to you in your/ her thoughts or understanding. Because आप (Aap) is very formal but sometime an indicator of more respect in the relationship.

मैं तुम से प्रेम करती हूँ
Main tum se prem karti hoon.

Note : “प्रेम” (Prem) could be heard as well. This word is less common in usage but could be found in Hindi literature and from the mouth of a Hindi Scholar. 🙂

2. Family or friends (Informal)

मैं तुम सब को प्यार करती हूँ.
Main tum sab ko pyaar karti hoon.
I love you all.

3. Family or friends (Formal)

मैं आप सब को प्यार करती हूँ.
Main aap sab ko pyaar karti hoon.
I love you all.

Note: For both 2. & 3., to family and friends, please note the use of  “को” (ko), which is usually preferred than “से” (se) as latter is used commonly in love relationships.