Hristov started his career in Levski Sofia, before relocating to Slavia Sofia in 2013.[3] He made his senior debut for Slavia in a match of the Europa League against Polish club Zagłębie Lubin on 30 June 2016, coming on as a substitute for Georgi Yomov.[4] A month later he made his Bulgarian First League debut in the opening game of the season against CSKA Sofia.[5] He scored his first goal on 10 September in a 5–2 home win over Lokomotiv Plovdiv. On 1 October Petko was joined by his twin brother Andrea Hristov in the central defense for the very first time against Vereya in the starting line-up.[6]
On 3 April 2017, Hristov signed his first professional contract with the club until 31 July 2020.[7]
Fiorentina
On 17 July 2017, Fiorentina confirmed they had signed Hristov on a five-year contract after Firenze Viola, Fiorentina owner, himself liked the player after seeing him in action at the European Under-19 Championship.[8] On 20 August 2017, he was on the bench for the first time in the Serie A match against Inter Milan.[9] He made his unofficial debut for the club against Real Madrid on 24 August coming on as a substitute in added time for the Trofeo Bernabeu game.[10][11]
Ternana (loan)
On 16 August 2018, Hristov joined Serie C club Ternana on loan until 30 June 2019.[12] He scored his first goal for the club on 4 December 2018 in a league match against Rimini won by Ternana by 3:0.[13]
Bisceglie (loan)
On 9 August 2019, Hristov joined Serie C club Bisceglie on loan until 30 June 2020.[14]
Pro Vercelli (loan)
On 18 September 2020, Hristov joined Serie C club Pro Vercelli on loan until 30 June 2021.
Spezia
After his good performance for Pro Vercelli in Serie C, on 5 July 2021 Hristov moved to Serie A team Spezia, signing a contract until 2025.[15] He made his debut for the team on 13 August in a Coppa Italia match against Pordenone and compleated his Serie A debut on 23 August in a match against Cagliari.[16]
Venezia (loan)
On 26 January 2023, Hristov was loaned to Venezia for the rest of the 2022–23 season.[17]
Hristov was described from the Italian media as a physically strong defender with great air abilities and being able to read the game which has led him to be compared to legendary Brazilian defender Thiago Silva.[24]